I love that the Ultima 4+5+6 trilogy is on GOG now. There's a great 256 graphics upgrade for Ultima IV out there, to be found here: http://www.moongates.com/u4/upgrade/upgrade.htm

Anyone know how I can upgrade my GOG version of IV with this graphics pack?

Since it says it doesn't work in dos, you'd probably not find it compatible with the dosbox deployment of U4 but you should be able to apply it and run the EXE directly, I think the GOG versions of games still have the original EXE as well as the EXE that runs the dosbox wrapper.

Not sure what that'd do to the speed of the game though, it runs lightning fast on even the most modest modern machines

Well there has to be an EXE to run in DOSBox. :P

Anyway, it's certainly worth a try on the GOG version. If it doesn't for whatever reason you can just get the 'clean' version of Ultima IV from somewhere else (what with it being free and all).

Thanks for bringing this to my attention, I might give it a go if and when I try out UIV

You could always run the graphics upgrade with xu4, if you wanted.
It's working nigh to perfect for me on linux with the GOG version of the game, so I would assume it would run just as well under windows.

There are instructions here:
http://www.ultimaaiera.com/blog/patches-and-updates-for-the-good-old-games-ultima-releases/.

Read the section on Ultima 4 and it should tell you all the steps you need to download the patch for DOS. Its a lot simpler than it sounds and worked pretty well for me using Boxer.

EDIT: From the same website there is this: http://www.ultimaaiera.com/blog/new-download-pixs-ultima-patcher/
It should save the work involved and patch it for you. Admittedly I've never tried it myself as its only Windows.