On the flip side of the coin, I've used OS4.0 on the same 240MHz 603 with 256MB, motherboard IDE and basic PCMCIA network and just vanilla AGA.
It works fine provided that you apply the same common sense you do to 3.x and remember that just because you are now using a CPU with up to 5-10x the clock speed of your 68K, your native chipset is no faster than it ever was:
1) You stick to the same rules you use for native displays that you do under 3.x. That is to say, don't use 256 colour dblPAL modes and the like. I ran a flicker-fixed 16-colour PAL laced display and it was fine.
2) Turn off all the gradient fills and related candy. They are designed for RGB mode displays. Hence they don't look so hot on a 16 colour display anyway and they do slow down rendering.
3) Turn off solid window moving/sizing. AGA/ECS is no faster than it was under 3.x.
4) Don't use anti-aliased fonts, for the same reason as (2).
5) Replace the default 4.x RGBA icon set with icons from 3.x. Again, they are designed for real RGB displays, not indexed colour modes on planar hardware.
If you apply these simple steps, you will find that far from being as slow as a tortoise, it's perfectly usable. OS4 cannot make your AGA display go faster and the default installation is heavily biased towards RTG.
Of course, just like OS3.x, the experience on RTG is much improved.
