Yes, thanks. All the gears turn and that is good to keep the oil flowing through the transmission, but the manual 4th, though it turns, the other gears are just freewheeling, as you say, so are not under a load, the load goes straight through for what I can tell having had to replace two poorly rebuilt transmissions on a big truck. All the gears turn but only one gear is selected and under load.
My issues on the 3/4 ton truck was that 5th gear (the highest gear on it) is a really small gear with just a few teeth, unlike 1st gear that is a giant cog with lots of teeth, and the 5th gear is under a lot of load due to this. The poor workmanship in my rebuilt trans became obvious towing. I did not tow in 5th uphill but would on flat ground at a low load, but it still went. Not really an issue on an auto since the TC will protect the transmission from too much stress, but if you have a manual then be careful in high gear (O/D).
Newer auto transmissions use planetary gears to turn the shaft (Im sure I am not saying this right) and those 3 gears are spreading the workload out so these autos can be more durable than a manual which still has one gear turn one other gear. IDK what the Fronty has, these are old designs, so likely are old trans designs too.
**so, for those still confused - we are talking about gears that are under load, and not concerned if the un-used gears are turning or not, unused gears that spin are not under load, do not see stress, will not create heat, will not wear etc. So, in 4th gear you are not using 1st gear, not using 2nd gear, not using 5th gear, so the gears are not under load, not getting worn, not creating heat. There is no 4th gear in a way as there is no need to gear down or gear up because the input shaft and output shaft are turning the same speed. There are other toothed cog things going on, but there is no gearing in 4th.