Unlike Jolaes (Re: M-321 rev3.1 386DX40 mobo: worth restoring? ) i will say that this mobo is faster than sis rabbit clock-to-clock, but does not overclock as well as the SIS chipset.
The best models are versions 2.4, 2.5, 2.6 - highly recommend - fast, stable and just right.
Manufacturer apparently went into cost cutting land with the later models.
But 4 mem modules only indicate some specific issue you are hitting. The chipset supports up to 32mb ram.

I checked again my motherboard... cache memory in bank 0 is missing (128kb). This means the only way to upgrade the memory is going from 4 mb (4x1mb) to 16 mb (4x4mb) in bank 0 only memory modules?

Welcome to the forum, Sofu1982!😀
Your motherboard has two memory banks consisting of four slots each.
To add memory you have to add four simms of identical size in a bank. No mixing and matching.
This is due to the memory bus being 32bits wide and each 30pin simm having an 8bit interface.
With many boards that have multiple banks you can use different sized simms between the banks (i.e. 4x256kb in bank1 / 4x1mb in bank2).

The cache banks are cache only. RAM has its own banks.

I hope this helps! 👍 If you need further assistance with your board please make a new thread though; most of this thread is 7+ years old.😉
Edit: Or just ask here now that it's its own seperate thread!😎

Split posts to new thread.