The basket need not be 4x4 if it is situated less than 10 tefachim above the ground. Rashi explains it's as if it is part of the ground. Why then can't we use this concept to make a suspended sukkah wall kosher at just less than 10 tefachim high and bypass lavud at only 3 tefachim? Why shouldn't this work?
Daniel Ettedgui, Boca Raton, USA
Lavud is a Din specifically concerning "Mechitzos," partitions or walls (while the Gemara you have cited is not discussing walls). This is clear from Eruvin 4a, where the Gemara states that "Shi'urim, Chatzisos, and Mechitzos are Halachah Moshe mi'Sinai." The Gemara examines each of these three Dinim, and on 4b concludes that "Mechitzos are Halachah l'Moshe mi'Sinai" refers to "Gud, Lavud, and Dofen Akumah." We see from there that Lavud is an example of a Din concerning walls. Rashi (4b, DH l'Gud) writes that Lavud means that something that is short is added on to and lengthened. The Halachah l'Moshe mi'Sinai states that the lengthening process works only up to three Tefachim. Consequently, if the Sukah wall is more than 7 Tefachim long and is hanging down at a distance of less than 3 Tefachim from the ground, the principle of Lavud works and lengthens the wall so that we consider it to be a 10-Tefach wall which now touches the ground.
However, the Gemara in Eruvin 33b is not discussing Mechitzos, but rather it is discussing where the Eruv is considered to be placed. The Din is that the Eruv must be situated on the ground. If it is within 10 Tefachim of the ground, it is considered as thought it is on the ground. Rashi (33b, end of DH Ela) writes that this is because "Kelutah k'Mi she'Hunchah Dami" -- if it is "absorbed" in the air within 10 Tefachim of the ground, it is considered equivalent to resting on the ground. This is entirely unrelated to Mechitzos, and therefore has nothing to do with Lavud.
Kol Tuv,
Dovid Bloom