I started experiencing the same problem after about two years and managed to solve it though it wasn't easy so be prepared to dismantle this thing down to the motherboard!
The first thing you should do is remove the brush entirely and then turn it on and attempt a clean (granted it won't actually clean of course without a brush!). If you still get the four beeps with no brush attached then you can safely rule out that this is a brush problem.
If it cleans without the brush but beeps with the brush in, you have yourself a brush problem which you can resolve following the many tips above.
If it's a non-brush issue, the bad news is, you'll have to get into the guts to fix it.
Get yourself a screwdriver and carefully remove all the screws from the underbelly. What you'll find inside is (hopefully) a load of dust/hair lodged in the wheel/brush motors and everywhere else (motherboard etc - mine was a mess). To be extra thorough I'd recommend taking the smaller screws out around the wheels/brush so that you can partially remove the motors. I "cleaned" them with my hands/fingers and a good ol blow like it's a NES cartridge (extra points if you get that reference).
Re-assemble with patience and with a bit of luck you'll be up and running in no time.
As before, test it without the brush to see if you fixed the non-brush issue. Hopefully you'll be able to rule that out and test again, this time with the brush.
This isn't for the faint hearted but as long your patient and keep an eye on everything you've done (take pics if you think it'll help) you'll be mighty satisfied when it springs back to life.
My Roomba which is two years older than my Eufy has never failed me like this. Goes to show, you get what you pay for. I'll avoid Eufy for my next purchase!