I'm testing this ECS UMB486V Rev. 2 with an AMD 486DX4-100. However, the board sees it as DX2-80. Funnily it doesn't have a 40 MHz setting for the FSB. Only 25, 33 and 50. Is there a trick to make this board work with the 100 MHz CPU? FSB of 33 should do the trick but it doesn't.
I also can't find any voltage settings on the board, you can only choose SX, DX/DX2 or P24T.
FSB 40 MHz Question
It seems just above (in the left when having the rear of the board on the bottom) of your jumpers block there is the clock gen chip.
But in the datasheet of the most likely used clock generator chip you find more possibilities: https://theretroweb.com/chips/6606
Where C=25 D=33,3 E=40 F=50 MHz. The clock gen interprets closed jumpers as 0 and open jumpers as 1.
So the jumper table for you is:
1 JP18 JP19 JP20 JP21 225 Closed Closed Open Open 333,3 Open Closed Open Open 440 Closed Open Open Open 550 Open Open Open Open
3.3 Volt Problem
But the problem with your DX4 is that this board does not have support for 3.3 Volt CPUs. You'd damage your DX4 when you don't use a voltage interposer.
BIOS Problem
The next thing you mentioned is that the BIOS displays your DX4-100 at 33 MHz FSB as DX2-80.
It will configure your chipset wrong because it does not know about a DX4. It will configure it with too many waitstates matching to a 40 MHz FSB instead of faster settings of your 33 MHz FSB.
Write back problem
It seems this board does not support CPUs with write back cache. So you'll lose performance benefits of a DX4.
Conclusion
I recommend you to go with a 5 Volt DX2-80 instead. Cyrix (Texas Instruments, ST, IBM) or AMD. You'd likely need a heatsink and probably a fan too.
