The standard X Server that comes with AIX 4.3.3 will run KDE no problem. IBM provides an AIX Toolbox for Linux Applications, with a wide range of GNU and opensource software in rpm format.
See their site:
It will explain the procedure better than I could regurgitate it here, though basically all you need is the rpm installp image and you will be set for rpm. Note that if you can, update to AIX 5L, which was designed for linux software compatibility, while 4.3.3 was a bit of a hack on their part.