If for some reason Basecamp (or in my case, Mapsource its precursor) doesn't suit, then my first response is "just use a text editor". However, what is the object of the exercise? Is it to:
a) replace one very large file with 4 smaller ones each approximately 1/4 the size of the original?
b) define some specific locations X, Y, Z between A and B so that you have files for A->X, X->Y, Y->Z, Z->B. ?
Other questions:
c) is the original a tracklog? (ie, do we assume you don't want to lose any data, even if it's spurious)
d) does it matter if thare are overlaps at the joins? (I think overlaps are often quite useful, certainly no real need to go to lengths to avoid them)
If it's (a) then
Phil's Simple GPX will do a good job, with the caveat that there will be some (spurious) data loss (because that's what it aims to do - remove 'fluff'). That aside, you can set it to split a track into n pieces, and define the distance interval to do the split. Opt for '1 metre accuracy' and retain timestamps and elevation, to minimise data loss.
If it's (b) then Basecamp is good (so I've heard, never experienced it myself as a satisfied Mapsource user).
If it's anything else but not (b) then it really is a remarkably simple job to do with a competent text editor, preferably one that shows you line numbers and can handle at least 3 files open simultaneously.