Not sure what the concern is, most of the time you use lock-up is when crusing anyway, that's only 50-60HP, ain't like your towing a heavy load or slamming at the drags.
Yes...very true!! Your car is under a light load when you're in 4th gear. And if 4th gear is engaged and has the full force of the front pump there is no reason why it should slip. But it has been documented that in order for the torque converter to function (lock up) it must use the same fluid that is used to keep 4th gear engaged. This lock up function takes some fluid pressure away from 4th gear. It's not a BIG deal as 4 speed automatics will normally last a very long time when they are not abused and when they are properly maintained. But no matter how good you are to your AOD 4th gear will always last longer if it has the optimum amount of pressure required to keep the clutches positively engaged. It comes down to a matter of choice. Use a non lock up which creates more heat requiring the use of an extra trans cooler (and lose a 2 or 3 MPG). Or use a lock up converter accepting the fact that it will take some pressure out of the system which inadvertently decreases line pressure to 4th gear.
Its' a choice.