The 4 GB limit is a RAM limit, not file size limit. I had to split my family files back when I was using FTM 2006 because the 2GB file size limit was reached. When I try to merge the files again in 2012, the program crashes. Both files, including media, is about 69GB now.
FTM will use available RAM memory until it crashes. This occurs somewhere between 1.3 and 1.5GB of FTM RAM usage. At this point, the OS, other programs, have used up all of the 4 GB of RAM. it does not matter what amount of RAM your system has (mine has 12GB), FTM will still crash under its current programming.
Please make sure you send a bug report to FTM (I have since version 2008 to solve this problem.) I'd think too that the program could warn the user of the coming crash before the merge actually happens. I used to have to wait overnight for the program to crash during a merge. So for now, the two files remain separate (my ancestry and my wife's ancestry.) You could say that FTM has caused my children's family to split :(.