Your board may be the same as the one in this thread Re: Another semi-unknown 486 board , which includes a BIOS dump .

according to https://www.wimsbios.com/forum/bios-utilities … sion-t5612.html

Thanks. What exactly do I need to put on the floppy?

Well, it seems that the chip is an EPROM (TMS27C010A-12 to be exact). Correct me if I'm wrong, but that means that I can't write on the chip without erasing it with specialized equipment. right?
I pulled the chip out and dumped its content with a USB programmer (G540). The content of the file is somewhat similar to the file dumped in the above thread, but not exactly the same.
Not sure what to do now... Can I order clean chips to flash?

I made some progress...
Found a flash chip from a Pentium motherboard and I was able to flash the KM-S4-2 BIOS on it. Problem is that it doesn't boot correctly.
When either the floppy or the keyboard are connected, it simply does nothing on boot. no beeps, no screen, no floppy seek. just fans spinning.
When the keyboard and floppy are disconnected, the screen does turn on and display the checksum error, following with endless fast beeps and a "system halt" message due to no floppy.
I was able to flash the original BIOS on the chip and it behaves the same as the original EPROM. I have switched versions several times now, and the behavior is pretty consistent.
I also tried clearing the CMOS memory but it didn't help.

I could be wrong, but I think the floppy it expects after "checksum error" is one containing the correct bios file, and not the ordinary boot disk.
That spare chip isn't actually needed, you could have used the original one after you saved its content first. The KM-S4-2 board isn't identical to yours, from the little I can tell from your very small photo. I would try other BIOSes of boards with the same SiS496/497 combo, I'm providing a few of them below, hopefully one will allow you to start your board but don't know what to say about i/o working if there's a different chip than the BIOS expects...

Edit: found a photo of your board on the net, it seems to have a Winbond i/o chip, so first you should try the LS486e and J446A BIOSes.

Thanks. I tried putting a bios file in the floppy along with AWDFLASH.EXE, with and without making it bootable, and with and without autoexec.bat but it didn't work.
I also tried a bunch of other BIOS files, as you suggested but I got the same message.
Again, as the original chip is EPROM (the kind with the glass window), It can't be written to without blanking it first, which requires exposure to UV light etc.
But I did save the original content. I'll upload it here for future's sake.

I contacted the seller and he drew my attention to the fact that the board expects FPM memory, while I used EDO... So it is possible that using the right memory would solve the issue.
I must admit that I never heard of FPM memory until today. I'll look in my pile of memory sticks and hopefully there are some FPMs there. I ordered some on eBay in any case 😉
Hopefully my flashing shenanigans did not damage anything.

Same problem, no solution:
Shuttle HOT 555 VER 1.41 BIOS

In the meantime I got this error on another board when a Dallas with empty battery is installed. With a new Dallas it works. Maybe you should check...

Good news! got my hands on some non EDO memory, and the motherboard boots! I'm installing DOS 6.22 as we speak 😀

I'm attaching the BIOS dump of this motherboard, just in case someone will ever need it.