Just have your 4L60E "built" for your application. You'll be able to tow in 4th
if it's built correctly.
Why would you want to tow in 4th? Especially if you're towing super-heavy?
Look at the construction of the transmission. Trace back its predecessors and you'll see it was originally a 3-speed and then evolved to a 4-speed when GM added overdrive by putting in a small planetary gearset. 3rd gear is a 1:1 drive ratio. 4th gear is a 0.69 drive ratio.
If you're towing in 3rd, the transmission has minimal stress; it's simply acting as a pass-through. The power is being transmitted through the strongest part of the transmission.
If you're towing in 4th, now you're transmitting all the power/torque through that small set of planetaries, AND they're having to multiply that torque as well because of the reduced drive ratio.
Personally, I'd rather send the power through the strongest part of a stock transmission than the weakest part of a built transmission.
But, then again, what do I know? I only have 325k miles on my weak, POS 4L60.