Old DOS and 486 systems are obscure and require an obsolete set of knowledge
Ah yes, one of my various areas of obsolete expertise
If you were to install a DX4 in place of a DX2, you may run into a problem with voltage or with write-back cache (which needs to be supported by the BIOS). That's what those adaptor boards that fit between the CPU and socket were for, to drop the voltage to 3.3 or 3.45V and disable the write-back cache. Then the DX4 or AMD 5x86 (and sometimes the Cyrix 5x86) can be used in an old board.
There is also the Pentium Overdrive chip (IIRC you need socket 3 for that, as socket 1 didn't have all the pins)
You'd have to try hard to cook a 486 chip. I ran a 3.3V Cyrix 486-100 (which has write-through cache) in a 5V board for quite some time before I learned that it wasn't supposed to work, yet it didn't have a problem.
40 and 50MHz VLB boards were not uncommon, though sometimes the higher speed settings were undocumented, and in practice there weren't as many CPUs around that officially supported those speeds anyway. But the 5x86 and Intel DX4 CPUs supported different multipliers, 2x, 2.5x, 3x, and 4x, depending on the paricular model CPU. I have a couple of VLB socket 3 boards that support 60 and 66MHz bus. I had a Cyrix 5x86-120 in there running at 60x2 with a Trident 9440 and it could do 50MB/sec. That combo wasn't completely stable though. Later I got a PCI socket 3 board and ran the Cyrix chip in there also at 60MHz x2 (had to divide the PCI clock down to 30MHz, else the onboard IDE would crap out). That one was stable, and the L2 cache was faster on it but the video and IDE was not as fast.
Your onboard video may be faster than what the ISA bus can manage. ISA cards max out around 5MB/sec. You can check with the program SVGASPD
http://www.hyakushiki.net/junk/memspd.zipI have at least one set of 4 4MB 30-pin SIMMs in my pile, as well as various 486 motherboards, VLB and ISA cards, and CPUs
