While updating from J!3 --> J!4 I have seen a recurring issue. I keep PB, TC and MobileMenuPro installed during the upgrade and they don't cause an issue to the upgrade. However, there's a folder under plugins/pagbuilderck/contact which is causing the frontend to crash. Renaming the first to "contact1" or, for that matter, just deleting it fixes the issue. Not sure what it does on J!3 sites, but J!4 does not like it.
A good landing is one you can walk away from.
A great landing is one where you can reuse the aircraft!
