I've installed the Ultima IV upgrade patch for the GOG version, I've done all the needed steps (even creating a character before installing the patch) and it worked as it was supposed to; that is, until it began to crash whenever I tried to enter Castle Britain.
I even re-installed the patch and the same problem kept coming, so I re-installed both patch and game, and they worked fine for a little until I had the same issues, but this time the game crashed in the castle even after I removed the patch.

I re-installed the game only and I'm wondering what I can do in order to fix that problem, I really want to play the game with MIDI soundtrack; I would try xu4 instead but for some reason that program doesn't work whenever I open it.

xu4 crashes? That's strange. Have you tried uninstalling Ultima IV, reinstalling a fresh install (without the upgrade patch) and then playing the game?

Have you alternatively tried using ScummVM? That is supposed to integrate xu4 into its featureset, and I think they've ironed out most of the bugs in that version. (Enhanced or not Enhanced is the same install...just Enhanced as some "Enhanced" features turned on by default instead of turned on manually)

What Upgrade patch are you referring to? Is this from Ultima Patcher 1.7?

On a lark I decided to download the latest version of the Ultima Patcher (version 1.7 here: https://www.pixsoriginadventures.co.uk/category/ultima-patcher/), and now I see your issue. When I enter the castle I get an error message "Opening LCB_2.SAV" message, and the game crashes out. I've attached a screenshot here. It's definitely the updater, b/c if I go into the command prompt and execute "switch.bat," then it goes back to EGA mode and I don't get the crash. But if I execute "switch.bat" again, it goes back to VGA mode and I get the crash.

It seems like when you use Ultima Patcher to install the 2.00 enhancements, it points to the wrong file. It points to LCB_2.SAV instead of LCB_2.ULT and to LCB_1.SAV instead of LCB_1.ULT. In order to fix this, I just created copies of LCB_2.ULT and LCB1.ULT and renamed those copies LCB_2.SAV and LCB_1.SAV.

If there are additional crashes, you may need to also rename other *.ULT files to *.SAV files.

The good news is that xu4 resolves this issue without needing to rename files. Getting it to work is a bit tricky, you need two zip files. One called Ultima4.zip that contains a zip of your Ultima 4 files, and another called u4upgrad.zip that contains the upgrade files.

Ultima Patcher v1.71 now fixes the issue with the "version 2.0" upgrade that caused the castle-crashing bug I saw in Ultima IV.

which can be found here:
https://github.com/Fenyx4/UltimaPatcher/releases/tag/1.71

Better to link the "releases" directory, so that anyone in the future who looks at this thread downloads the latest version, instead of version 1.71 (assuming that additional versions will be released in the future)

OH LORD! How come I didn't see this when you posted about?! THANK YOU SO MUCH! Sorry for the late answer, life's gotten pretty rushed and I barely got any time left to fix Ultima 4; but thank you again for posting aobut version 1.7, I didn't even know it was out You're a life saver!

THANK YOU!