There are two stories about the use of memory by KDE4 vs the use of memory of KDE3, they have somehow different information, actually opposite.
The two “studies” comes from the same author on its first test, he used KSysguard applications to measure the amount of memory used by both versions of KDE and the results showed that KDE 4 uses 40% less memory than KDE 3, this story is now on Digg and Slashdot.
It is even on reddit, pointing to the slashdot article, well this making everybody think that KDE 4 with composite on uses less memory than KDE 3.5, the same author of the first post, made another one as a correction to the first one, this time he used exmap to measure the memory used by both KDE 4 and KDE 3, this is a really good test, but now it shows that KDE 4 uses more memory than KDE 3.
KDE 4 requires more memory. In similar conditions KDE 3 consumed 97 MB on memory, whereas KDE 4 about 170 MB
but take into account that KDE4 has composite on, it will be great if someone could make a test in similar conditions, with composite turned off.
Here are the two original posts.
first one (Polish)
second one (English)