As you know, Opencart 4 has a built-in upgrade function. But the fact is - it is not working at all.
Oh, if that person that wrote that code (whomever it is) had tested it a single time - but he didn't.
I can't imagine how this can happen at all!

Anyway, here is the fix. Actually, it took 12 fixes that any normal programmer will not be able to carry just to upgrade the single store.
But the fact is with this fix the Opencart 4 upgrade should execute ok just by a few clicks in admin.

Instructions
1. Upload mx_oc4_upgrade_fix.zip contents to your admin folder rewriting files
2. In admin go to System -> Maintenance -> Upgrade
3. If you do not see "MX Upgrade" title - you have done something wrong when uploading files
4. Start the process and it should go relatively fast and smooth.

If you face any issues - please report to me, that should be fixed )