As we learned in shiur, there is a machlokes if a reshus hayachid requires הנחה ע”ג מקום ד על ד or not. We learned the simple peshat of 8a that both Rebbi and Chachamim agree with Rav Chisda that indeed a reshus hayachid does not require it. This is the way the Rashba understands as well, though in doing so he takes us through all the sugyos from 4b until 5b showing that we need to check each one as many of them are not easy to understand in this light. This provides with a closer analysis of the cases and how to understand them and a chazara over those cases.
As we remember on 4a, the gemara asks – don’t we require both עקירה and הנחה from a makom 4x4? The first proof, that it is shitas Rabbi Akiva does not pose a problem since his example involves a hanacha in reshus harabim and thus we can say that this is not an issue within reshus hayachid. However, in the first suggestion of shitas Rebbi, the issue of the זיז would depend on how we understand the case of שדי נופו בתר עיקרו. According to Rashi, the tree is considered to be in reshus harabim, so again the hanacha comparison to our mishna is fine and does not require 4x4 in reshus hayachid. (See tosfos for alternative peshat)
However, as we continue, the Rashba says it would seem that the sugya on dapim 4-5 really does not agree with Rav Chisda, as we see from the טרסקל case. The gemara there explicitly says “התינח טרסקל ברשות היחיד” which clearly shows a need for a טרסקל even in reshus hayachid for an area of 4x4. Similarly, in the previous gemara, the second shitas Rebbis is one where the hanacha took place in a reshus hayachid, meaning that Rebbi is the one who holds you dont need 4x4 in a reshus hayachid but others disagree, meaning he is the one shita that holds this way. Also, the case of הכניס ידו לתוך חצר חבירו וקיבל מי גשמים, here too we have a case in reshus hayachid and the gemara asks where is the 4x4?
However, the Rashba deflects all these and explains that the entire sugya is really only asking in one directions – about the case of placing in the hand of the עני and not the other way around. And the focus of the sugya is never on reshus hayachid.
A few other interesting opinions about reshus hayachid are: the shita of the Rach (quoted in Ramban) that only above 10 does not require 4x4, so therefore Rav Chisda is not taking on the sugya on 4b-5a. Also, a difference is noted between a person – who is not the reshus itself and thus would require 4x4 (just that in the maskana the hand is good enough) as opposed to the reshus itself or objects in it like the amud.