Hmm, I did some testing on the way in to work this morning. I wasn't towing, but it appears there's no way to run 4000 RPMs in 4th in an automatic at that speed, it shifts to 3rd before it climbs that high. 70 MPH in 3rd with the converter locked is about 3600 RPM, and 3rd unlocked is 4000 RPM. 3rd unlocked is pretty rare, about the only time I get that combination is if I turn the overdrive back on while I'm still hard on the gas under load, it won't shift to 4th until you back off a bit. If you're spending a lot of time in 3 unlocked, I'd be surprised. It's better than 4 unlocked, but still not something that should go on for long.
Brent