On the question below couldn't we anwer that according to Rashi the ledge doesn't need to be 4 Tefachim wide to minimize the height of the wall because a ledge is part of the wall . However a object which is placed next to the wall needs be 4 tefachim wide (and tall) to dimiinish the height of the wall ?
>> 2) THE LADDER MUST REACH THE TOP OF THE WALL
QUESTION: We see from the various opinions in the Gemara (see previous
Insight) that the ladder must reach the top of the wall (or within 3
Tefachim of the top) in order to permit using the top of the wall and to
permit the two Chatzeros on either side of the wall to make one Eruv
Why is that? The Gemara earlier (77b) discussed diminishing the 10-Tefach
height of the wall by placing a platform (Itzteva) on the ground next to
the wall. The platform certainly did not have to reach to the top of the
wall! Similarly, the Gemara earlier on this Amud said that a ledge (Ziz)
protruding from a wall can also decrease the 10-Tefach height of a wall.
Why, then, does a ladder have to reach to the top of the wall? <<
The Gemara (77b) says that a ladder which has a bottom rung which is four or more Tefachim wide is Mema'et from the height of the wall (even though it does not reach the top). Here we see, then, that even though the ladder is not part of the wall, it is Mema'et, even though it does not reach the top (we actually should have included this case in the question!).
Tosfos and the Ritva give a third answer b'Dochek and say that when the ladder is not four Tefachim wide, then it must reach the top. The ladder on 78a must be one that is not four Tefachim wide. When it is four Tefachim wide, it does not need to reach the top (like the Itzteva, Ziz, and the ladder on 77b).