I have a Mini too, and I could see myself get a MK4 as the Mini has been a bit of a disappointment. The X and Y axes on mine aren't 90° and there seems to be no hardware adjustment (just a rigid printed part), no support for software adjustments either (the respective GCode is disabled in firmware), so I can't print proper rectangles, just parallelograms, and any boxes and lids I print never fit properly. I want to keep tinkering to a minimum (that's why I got a Prusa), and I can make do with goskew and preprocessing the gcode, but I found I have to repeat callibration quite often and it's a fairly annoying process and not very exact and means I can't reuse gcode files at a later date, so having a printer that doesn't have this defect would be very welcome.

My Mini also likes to ram the nozzle into the build plate once in a while without any of it's safeties triggering, heating element stuck on and the Z stepper fully engaged, so I can't let it run unsupervised either, and the filament sensor is pretty hit and miss as well. I may have had bad luck with my unit (I ordered right at release), but I'm definitely looking for something more reliable and, frankly, less dangerous.

The Prusa Mk4 does look like a solid hands-off machine and hopefully they didn't cut corners there like with the Mini, and hopefully customer support is also better for the more expensive ones. I don't have any use for the XL's feature set (I only print relatively small parts and not that many), but I didn't want to get a Mk3s as it's been pretty dated for a long time now. I'll wait a bit until other people have had a chance to run into any major flaws though.

Very happy early Prusa Mini owner. I do remember some difficulty when assembling it, which lead to non-square axes. After an hour or so I eventually got right, so you might want to try redoing the tower assembly.

I don't have a filament sensor (I also print mainly small parts, so rare to hit the end of a spool), and haven't had any problems with heating elements, z steppers or nozzle crashes. I did experience the extruder clicking due to shrinking PTFE in the hot end, but I've ordered the E3D Revo mini hot end which will prevent that problem ever happening again.

I had clogging problems on mine - three or four different fixes to sort it out, and the thing that finally nailed it was replacing the PTFE hotend lining with Capricorn. It's been fire-and-forget since then.