The 4-speed functions better as you drive the vehicle, and better
matches engine RPM to load, thus giving better performance and fuel
economy.  However it is very sensitive to the fluid you use.  Put the
specified ATF+3 in it, or better the ATF+4, and change fluid regularly,
and it will hold up as well as anybody's typical automatic transmission.
 Put Dexron in it and you will kill it, no matter what any number of
self-proclaimed gurus at the quick-change oil places say.

The 3-speed, as mentioned, is less efficient but its durability is
better than typical automatics.  It's derived from the near-bulletproof
Chrysler Torqueflite transmission.