XSurf100 usb module is one of the the three things missing on my Amiga4000T. 
The other two are hardware accelerated audio and video playback. (An upgraded version of a card like Peggy+.. Preferably both audio and video in one card since I don't have enough Zorro slots available)
I was going to comment "just get one of these":

Then I realized you said A4000T, not A4000. And then I realized the darn thing doesn't include Zorro slots, anyway. Gah, need more coffee!

Is that new Prisma sound card going to offer anything you're looking for? I haven't had a chance to catch up on it lately. I'd love a new sound card for my A2000 but I don't see that it's going to offer much over my AD516, if it can't do AHI it's basically just a fancy MP3 player, right?