Did those 4MB SIMMS come with A4000 Rev B? If yes we can assume they should work.
Those 74LS245 chips between CHIP RAM bank and daughterboard bank that are one of the hottest ICs when looking operational A4000 Rev B with thermal camera. Possibly because of this heat one of those chips may fail. I know at least one case where changing one or more of those ICs solved all motherboard Fast RAM issues. So my point is that it may not be only U891. He used this chip as replacement:
http://fi.farnell.com/fairchild-semiconductor/74f245sc/74f-fast-ttl-smd-74f245-soic20/dp/1014088?Ntt=101-4088One other case I have read about had failed clock latch chip (possibly killed because of excessive heat while desoldering battery). Also causing Fast RAM issues.
In the past my motherboard had a cut in traces under BANK 4 caused by battery leak by recless previous owner. If it looks like battery has never leaked this is ofcourse not your case either.
My experience is that EDO simms are used in A4000 motherboard it may see only one fourth of the SIMM. For example 8MB EDO shows 2MB RAM. EDO SIMMs work in most accelerator cards however. But I assume your memory is not EDO if it came with the motherboard.
I can also verify adding 8MB SIMM memory to BANK 1 and BANK 3 is correct. I use similar setup for my two A4000 motherboards.
Not really helpful but hopefully you find the failed part eventually.
