I think that 4GB is "JUST ABOUT ENOUGH" as long as absolutely everything is then installed to some other drive.
I use C: for Windows only, and only some system apps are put to C: and everythign else goes to D:, I have just had a look at how much diskspace is used up on C: and I am using up 3.4GB.
Given that I have a fair load of system apps like Nero, O&O, PAgeDeFrag, Antivirus & Anti SPyware apps and so on all on C:, including a small pagefile ( 256MB ), had I installed what I could to D: instead of C:, I would have saved myself even more.
I cant wait to do my own tests... With Linux booting up a lot slower than Windows does, I will be trying out Linux before Windows, with root on this, and /home on a real HD... I recon that will be useful.