"Using memoryadresses [sic] from more that [sic] 16GB of memory"

I set my pagefile to be 1.5x my RAM

The problem is that it's not even getting anywhere close to touching swap (especially considering the 24GB of physical memory). Unless Unity bases this limit on the pagefile / swap partition size (which would be very silly)?

Unity 4 is 32 bit only. But I don't get why it is limited to 16 gig then. When 3.5-4 gig is the max anyway.

I wasn't aware that it was 32-bit only, seeing as the two Unity-based games I've seen this happen in ($$anonymous$$erbal Space Program and Cities: Skylines) both have 64-bit executables, at least on GNU/Linux (and, as far as I know, both use Unity 4, with - IIRC - C#). This is also very different from a 32-bit memory exhaustion error, in my experience (having run memory-heavy 32-bit software in the past).

Whatever the case, it seems to be an inconsistent error, as I'm finding out lately with Cities: Skylines; the Using memoryadresses from more that 16GB of memory error will often fire even when the program has only used 14GB of memory, and at other times won't fire even after memory use has exceeded 16GB.

The Unity 4 editor is 32-bit only, but that's unrelated to builds, which have had the option for 64-bit for years.