I use SF9 to mix four tracks down to 2 using the "channel converter" function.
Put all four of your tracks into a single quad track file, do any alignment needed. Then use the channel converter by selecting a percentage of each source (file) to go to either output 1 or output 2. (You must have the channel converter set for two output files.)
Example;
40% source 1 to output 1, 0% to output 2
0% source 2 to output 1, 40% to output 2
60% source 3 to output 1, 0% to output 2
0% source 4 to output 1, 60% to output 2
In this case sources 1&2 and 3&4 would be stereo files with 1&3 being the left channels and 3&4 being the right.
You can then sample the mix, although it only lets you listen to about 10 seconds.
I then just play around with the percentages until I find a mix that sounds like what I am after, then press OK to mix the tracks.
Kind of awkward the first time around, but it works.
JD