In general, 486-era systems were far less choosy regarding RAM brand/model than has been the case the past 15-20 years, which is why brands are less commonly mentioned.
RAM density and structure was very relevant. 486 memory controllers (outside of obscure server designs) were 32b, so you minimum unit of RAM is also 32b wide. That means 4 30p SIMMs or 1 72p SIMM. Now, some 386 chipsets did memory interleaving, but iirc it was never a 486 thing, so there is no performance benefit to be gained from multiple memory banks (multiple SIMMs). Theoretically a single bank loads the bus less than multiple banks so you can use tighter timings on a single 16MB SIMM vs 2x8MB or 4x4MB. In practice it's not likely to be at all relevant at the low 33MHz bus speed of your DX4. The only remaining consideration is upgrade space: if you have four 72p SIMM slots and you're only using one, that gives you three more to play with later.
A standard recommendation back in the day was to match the metal of pins and pads, so if your SIMM slot has gold pins, go for SIMMs with gold pads, if it has tin pins, go for tin pads. Supposedly that would reduce corrosion. >25 years later nobody has actually observed more corrosion on mismatched metals, so take this a seriously as you feel like.
Finally parity support depends on motherboard & chipset. Some can't use it (but it doesn't hurt either), some can use it, a few absolutely require it. Looking here all combinations are listed as "x36" which implies parity. I would be surprised if it is a hard requirement, but there's no reason not to get parity SIMMs (worst case you just don't use the extra bit), so if in doubt, get parity.
As for where to get it... I usually get mine out of bigger hauls locally, but occasionally I have bought stuff on eBay (very specific things like 16MB 30p SIMMs or 128MB 72p SIMMs) and generally had good experiences, just be absolutely sure what you need and triple-check SIMM numbers to be sure that's what you're getting.
