A4000 and A4091

[amiadudeorwat]

The A4091 doesn't need rev 3.2 A3640, 3.1 should work just fine.  Also the problem is the Motherboard RAM/A3640, it was designed for the A3000 so the crippled A4000 Motherboard memory with the A3640 is at fault for the 4-5MB/s speeds.  Remember to set Synchronous mode in the RDB and if possible get some Z3 RAM.  With an 3128 or something like it with higher priority than motherboard memory you can get 7MB/s.  But since the 3128 is slower still than A4000 motherboard RAM for CPU access the rest of your system slows down slightly.

[amiadudeorwat]

It could be your drive is taking too long to spin-up.  Try flipping dipswitch 5, this gives it more time to spin the drive up.

DIP switch settings        
switch 1-3- SCSI ID        
switch 4- fast bus: ON - disabled        
switch 5- delayed autoboot: ON - enabled        
switch 6- synchronous mode: ON - disabled        
switch 7- termination: ON - disabled        
switch 8- LUN: ON - enabled

[amiadudeorwat]

I don't think it's a hardware problem.  Could you post your system specs again currently?  Are you using Aztec Monster, are there any IDE devices?  Did you try PFS-AIO?

[amiadudeorwat]

I have tried some configurations, my A4000 is recapped, a3640 is v3.1, buster 11 and 3.1 rom (original not flashed).

I not use Ide devices

Have you tried with the dipswitch 7 termination off?

[amiadudeorwat]

Yes I have external active terminator

And an internal one?