The 4 section one I believe is a little smaller for portability. That was the only reason but I may have to rethink.
I considered that, in looking at the RRS TVC-33 vs. the -34L. While the 4-section is 1.5" shorter, I decided that 1.5" wasn't enough - 25.5" (TVC-34L + leveling base) was still too long to be 'portable'. I did get a RRS TQC-14 + BH-30 as a travel tripod (it can still hold my 600 II), with the head on it's 20.5" long and when in its quiver bag, the whole thing still fits inside my Pelican Storm im2500 carry-on hard case - that's my definition of 'portable'. The TVC-33S would still not be quite short enough, and would have me hunching over during use (I'm 5'7"), whereas the -33 gives me a few inches extra for use on a slope.