Put KDE4.2 on my laptop this week. After killing my KDE3 setting by forgetting I moved KDEHOME, I undid most of that mess and got KDE4 set up in its own settings directory (by basically not setting KDEHOME – there appears to be no other way, which is silly if you ask me).
So, I did some browsing, worked in Konsole a bit and opened up some files in Kate. Then I hibernate to disk (with cache dropping enabled). 850MB of data was copied. This is compared to the easily <550MB I usually get with KDE3 with more websites and files open as well as Thunderbird.
Looks like I won’t be using KDE4 on my laptop any time soon. (Side note, my laptop uses the nv video driver, so I have no graphics acceleration, and I had all desktop effects disabled in KDE4)