There is a way to install greasemonkey on MAC OS X Safari. But I didn`t find a way yet to install it for Windows Safari 4.
Do you know a way to do it?
GreaseMonkey on Safari requires GreaseKit which is currently only available for Mac. I can't see any reference anywhere to this being ported or made available on Windows.
If font rendering is the reason you want to stick with Safari, just make the Windows fonts look like OSX fonts with GDI++
Here are some instructions on how to get this working on your computer:
Enjoy this beautiful font-smoothing.
The application might perform poorly and I can't guarantee anything though several sites seemed to promote it. I first wanted to link another site, but part of the instructions were Japanese and I had no idea what they were doing there. The current instructions are very straight format and can easily be sandboxed to try out. I hope no Japanese feelings were hurt by the previous version of this answer