You are correct in that the SX4 shares the same chip. You might even be able to flash your SX4100 with a SX4's BIOS! Unfortunately said SX4 also lacks Win98 drivers. I did check your assertion that they existed, but I could find no evidence sorry.

Nevertheless. The SX4100 may still allow you to boot into Win98. It will just likely be resigned to Int13h operation, much like posts on here regarding the Promise S300 TX2 or TX4.

I see. When I read the sx4 works in win98 I thought it was normal not ms dos compatibility mode. I actually don't have the sx4100 to try it out, I saw it on sale and wondered if it was worth getting. Upon further searching I don't think i can easily find the bios to flash it either unless jbod is built in. I'd experiment if it was cheap but it'll just be a boat anchor if it doesn't work.

Both SX4 and SX4100 have JBOD according to the manuals. So yep, should be fine. But yeah, limited to dos compatibility mode for 9x.