Can you provide more details - like how fast - miles per day you hope to travel, terrain type (mountains, hills or flat land)?
Here is my limited experience: you can travel many miles in 5S running 70 mph on flat land. Rolling hills 4S works better. 4S requires that you let up from the throttle to lock the converter. Locked, speed in 4S runs out well in the 55 - 60 mph range. Unlocked, the converter seems to stall at 2000 RPM which will hold long steep hills around 45 mph.
Most folks on here that tow have installed AUX trans coolers. I just unpacked mine yesterday and plan on the doing the install in the next couple weeks.
You should not be disapointed if you make the switch. :driving: