Forum OpenACS Q&A: AOLserver 4.x for ACS 3.4.9
Using AOLserver 4.0.x or 4.5.x will probably work for ACS 3.4.9, but you might have to tweak a few things. AFAIK, all the ad13 features that you should need to run ACS were eventually forward-ported to AOLserver 4.0.x. I don't know whether they kept the exact same API or not in all cases though, particularly for the character set stuff.
I don't think 4.0.x includes the ad13 Tcl bytecode cache, which might be a performance issue if your site is very busy. Then again, some very large sites (like Gustaf's) are running on OpenACS + AOLserver 4.x, and they don't seem to have missed it.