Actually, I too have needed to install qt-4.x on Slackware 11 and I did so without any major issues. I installed it to /usr/local just to make extra sure nothing is overwritten ... remember that kde still depends on qt-3.x, so don't remove it if you want to keep using kde apps.
Also, know that it will take a while to compile .... a really long while
(even on a 3.06 Ghz P4 HT) I think it took at least 45 min to 1 hr.