Kinyan Daled Amot
Portable items (mitaltelin) require a physically demonstrative action to entail an act of kinyan. Smaller items must be lifted (hagbaa), while larger items may be pulled into the reshut of the lokeiach (meshikha). The heaviest objects (such as boats or massive animals) may be acquired through mesira - merely transferring the reins from seller to buyer.
Ideally, hefker items should be acquired in accordance with these guidelines, but this would likely lead to violence, as competing individuals would attempt to execute the FIRST kinyan and enjoy the windfall of hefker acquisition. To avert these dust-ups, Chazal instituted a kinyan of daled amot (Bava Metzia 10a): The first person to be stationed within a four amot radius of the hefker item acquires the item even without performing a classic ACT of kinyan. Awarding the item to the first arriver without need for a physical act of kinyan naturally prevents altercations.
At first glance, this is a limited takana intended to solve a very specific problem. However, this narrow view is challenged by a gemara in Gittin (78a) which appears to extend the four amot rule to situation of divorce. The mishna describes a situation in which a husband hurled a get in the direction of his wife. If the get lands closer to her, she is divorced. R. Yochanan interprets the mishna literally, as describing a method of divorcing a woman simply by lodging the get in her GENERAL proximity. Rav, however, reinterprets the mishna, arguing that the woman is divorced only if the get lands within her four amah radius. Just as four amot facilitate the acquisition of a hefker item, they enable a necessary kinyan upon the get by a woman undergoing geirushin.
This extension of the four amot takana seems odd. Items within 4 amot have not undergone an act of kinyan, but rather have been unilaterally awarded to the first arriver to prevent hostilities among different people vying for the item. Since such hostilities will not occur between a man who wishes to divorce a particular wife, four amot should have no relevance.
The Ramban in Gittin senses this issue and asserts that a SECOND and separate takana evolved. Just as Chazal instituted the takana of daled amot and awarded hefker items to first arrivers, they similarly validated a divorce in a situation in which no kinyan was performed but the get was delivered into the four amot radius of a woman. Concerned with preventing aguna scenarios, Chazal were armed with a principle known as afkainhu, which allows them to validate get situations that would not otherwise be effective. Although the gemara in Gittin does not explicitly invoke afkainhu and the conditions in Gittin are not those that typically enable afkainhu, Chazal intervened nonetheless and validated an otherwise ineffective get delivery. No classic kinyan has been executed, yet the woman is divorced once the get reaches her daled amot.
The Ritva in Gittin quotes his teacher, the Ramban, but is dissatisfied with his explanation. He suggests a very different explanation of the gemara in Gittin, and in doing so revolutionizes the notion of daled amot. He argues that the reason the Chakhamim instituted the takana of daled amot was to prevent hostilities between two potential acquirers of an item, but they did not merely award the item to the first arriver; they REDEFINED a new form of kinyan. Although kinyanim are normally enabled by active demonstrations of baalut (such as meshicha, hagbaa, etc.), often, mere placement of an item in the reshut of the recipient is sufficient. This is referred to as kinyan chatzer. Perhaps Chazal in attempting to prevent hostilities rezoned a four amah radius as a persons temporary CHATZER capable of facilitating kinyanim just as a personal reshut can.
According to the Ritva, the extension of four amot from the context of hefker items to gittin clearly indicates that the takana was not NARROWLY defined as awarding the item to the first finder, but rather re-landscaped a halakhic area of daled amot into a chatzer. Once rezoned as a chatzer, these four amot can operate outside the parameters of hefker; they may also assist in performing a kinyan necessary for gittin. Just as a husband deposits a get into his wifes actual reshut (as described in the Torah), he may deposit it at least according to Rav into her four amot.
A similar question regarding how far the four amot takana may be extended arises in an interesting debate among the Rishonim as to whether four amot may be utilized to enable a sale, or even a gift. Presumably, since these are targeted kinyanim rather than a hefker free for all, the chance of hostilities is diminished, if not eliminated entirely. If Reuven wishes to gift the item to Shimon and not Levi, the latters act of kinyan would be ineffective, and Levi would therefore presumably not battle with Shimon over this kinyan prospect. Indeed, the Rashba in Gittin cites many Rishonim who inferred from the gemara in Bava Metzia which discusses about daled amot and hefker items that daled amot would NOT facilitate mekhira and matana. The Rashba himself, however, is incredulous. He maintains like the Ritva in Gittin that the takana rezoned daled amot as a personal reshut and once designated as such, these four amot can be employed for multiple functions. Just as the daled amot may be used to accomplish a get, they can similarly be utilized to facilitate a sale. In fact, a sale is a halakhically more powerful kinyan than hefker acquisition, since it is driven by the daat makneh of the seller. As such, a kinyan that works for hefker acquisition CERTAINLY is effective for sales and gifts. This position of the Rashba, of course, assumes that daled amot is truly a new form of kinyan.
An interesting question raised by Tosafot in Bava Metzia (10a) may indicate that they also inclined to view daled amot as a newly zoned chatzer. Tosafot cite a well-known gemara in Ketuvot (32a) describing the attempt of a ganav to execute a maaseh kinyan, an action necessary to perform the crime of geneiva. In dragging the item into a reshut ha-rabbim, the ganav has not yet performed meshikha or hagbaah and is not yet considered a ganav until he reaches his own reshut and meshikha is accomplished. Tosafot question why he isnt considered a ganav once he draws the item into his own daled amot, long before he reaches his actual property. From the fact that they pose this question, it is evident that Tosafot consider daled amot a rezoned chatzer that is available for a broad range of kinyan performances, including geneiva. If they viewed daled amot as a narrow awarding of chefetz to the first arriver, the entire question would have been illogical.
What remains unclear, however, is Tosafots answer. They appear to claim that a ganav cannot employ daled amot to finalize his geneiva because the Rabbanan only instituted daled amot to prevent hostilities regarding hefker items or to facilitate a get and avoid aguna situations. It thus appears that Tosafot REJECT the Ritvas and Rashbas broad definition of daled amot, choosing instead the Rambans narrow definition of two parallel but different takanot.
Interestingly, the Shach (Choshen Mishpat 243:9) offers a very different and more complicated reason to justify the failure of daled amot in the case of a ganav. He may have viewed the takana in the same broad strokes as the Ritva and Rashba and therefore felt compelled to offer a different justification for why daled amot do not work for a ganav.
Perhaps the most ambitious extension of the scope of daled amot is found in the Geonim, who questioned whether daled amot can be used as the basis for a kinyan agav. Agav allows acquiring minor mitaltelin items as throw-ins to major land acquisitions, even without performing an act of kinyan upon the mitaltelin. This is an extremely powerful form of kinyan and very useful in situations that do not easily allow for meshikha or hagbaah. However, only those who own land can initiate an agav, severely restricting the scope of this kinyan. Many Geonim suggested that since everyone theoretically owns his current daled amot (at least temporarily), anyone can utilize those daled amot as the basis for agav applications. According to this argument, Reuven can offer Shimon to transfer his daled amot to Shimon and throw in a cow without Shimon performing an act of kinyan upon that cow.
Without question, these Geonim agree with the Ritva and Rashba, who broadly defined daled amot as a rezoning of a daled amah radius as a temporary personal chatzer. Of course, that does not mean that the Rashba and Ritva would not necessarily agree with the Geonims conclusion. Just because daled amot has been re-designated as a chatzer and can facilitate kinyanim on hefker, gittin, sales, and (in theory) even geneiva does not mean that a person actually owns those daled amot sufficiently to manipulate the land as a basis for kinyan agav. (This question rests heavily upon the nature of kinyan agav, which is beyond the scope of this shiur.)
 See http://www.vbm-torah.org/archive/metho61/08agav.doc for an explanation of this intriguing kinyan.