AOLserver 4.0 beta 2 has ns_charsets, ns_encodingforcharset, and ns_startcontent, 3 AD13 C API funcs needed for OpenACS.  It is missing the charsets.tcl file from 3.3+AD13 and a few matching changes to init.tcl and http.tcl.  The ACT's Mark Page has got adding these on his To Do list.  I'm pretty sure you could just drop those files from 3.3+AD13 into 4b2 and it would work - i.e. I'm not using i18n, but I've installed the OpenACS HEAD that way.  Jeff Davis said that OpenACS 4.6 will install on AOLserver 4 without those Tcl procs.

I made some patches for 3.5.1 starting from work done by Pierre Asselin for 3.4.2.  You can find the results at
http://empoweringminds.mle.ie/openacs/ad13/  I'm not actually using those sources and would recommend going with 4.0 instead.

I posted a list of remaining AD13 vs 4.0 issues to the AOLserver mailing list the other day and got a few responses from OpenACS'ers.  In addition to the above:

1.  The ns_uuencode patch #474259.  I should submit an updated diff. OpenACS falls back to a Tcl proc if this patch isn't there.  The AD13+OACS implementation is broken.
2.  AD13 has byte range request support (a HTTP/1.1 feature).  There was a concern that, if added, this feature should be made configurable to prevent abuse by download accelerators.
3.  Small additions/bugfixes in AD13.  I'm not sure which of these are needed: ns_db stats, WarnEmpty parameter for database pools, SIGSEGV fix for Linux, 401 redirect in nsd.tcl fix, etc.
4.  Don reports that you can't virtual host two OpenACS 4.x sites currently because nscache maintains one giant global cache for everyone, so the two instances clash over their "util_memoize" cache.