>> I was upgrading from a prior 4.3-RELEASE, with a bit of 4.4 RC's in there
Never do binary upgrade, compile from src at all time. If you read my other post Setting up ports you should realized by now how unreliable binary upgrade could be.
>> I've got files from all sorts of releases in the system
Unlike Linux, in *BSDs, that shouldn't happen at all. When it comes to upgrading, *BSDs beat Linux in million times all the way.
>> which I had previously upgraded to 4.1.0 with no problems
Check and make sure this line -> XFREE86_VERSION=4 is in /etc/make.conf. If it's not already there, wipe out all installed ports and do a clean buildworld and go for 4.4-STABLE, then install X afterward. As I mentioned dozen times in other posts, there are at least 100 essential steps you must first do before installing X, if you don't take my suggestion, you are asking yourself for all these troubles.
>> When I upgraded BSD, all I upgraded was the kernel
When you upgrade, that's so-called buildworld in *BSDs. You need to build the entire system as every piece relies on the others.
>> I'll try installing KDE from the ports collection with libpng 1.2.0, hopefully it will work
Good luck. But I doubt that will be reliable and durable. More than likely, something is going to break again very soon.
BTW, have you ever cvsup'ed your ports tree?