Significantly more than the 4 MB requirement for AmigaOS 3.
Seems the culture of keeping things slim went out of the window.
Not really sure what you're looking for here. If you stripped it down to the level of functionality of OS3 then it wouldn't need anywhere near 128MB (and yes, I think 128 was recommended - it'll boot in 64 as someone else said). But then why bother with OS4 at all? And, for the record, OS3 (as in 3.0 and 3.1) would happily boot in 1MB.
There are a few modest eye-candy additions in OS4, but most of the memory footprint is pretty legitimate, and it can be reduced by stripping out modules if you're that way inclined. For example, the Kickstart supports a much broader range of hardware, including graphics cards and USB, and so has greatly outgrown the original 512KB ROM. Since the Kickstart is loaded into RAM, there's more memory used already than OS3 needed to boot. Then the larger screenmodes eat memory - a 32-bit 1600x1200 screen takes almost 100 times the memory that a 16-colour 640x256 desktop takes. Then take the integrated TCP/IP stack, new filesystems with much larger buffers for modern drives, CD-ROM support from Kickstart and so on. I think these are all quite legitimate uses of extra memory, and couldn't be considered bloat... It's probably as slimline as you get while still supporting the features it has, and I haven't checked it out, but probably compares well to a Linux distribution with a similar range of support.
I have still to find the CPU that will load anything from any disc storage by itself. Ie, all computers have some kind of startup "ROM". The only thing close is some ARM CPUs that has a circa 2 kB builtin ROM that will serially download code from I2S based EEPROM.
Sorry, I didn't think you were looking for those sort of specs, since it's never an issue. I don't think Windows lists a BIOS as a requirement either... Basically, any machine that an OS4 licence is available for has the required ROMs. On a classic it can be bootstrapped from the classic OS, so whatever Kickstart is in that machine is all you need. Not sure what size UBoot is but it's available for download for some of the platforms so that would give you an idea.
