A/T - No power in 4th

I live in a hilly area and noticed my car really struggling to make it up hills and narrowed it down to it being in fourth. If I switch to the manual shifter and put it in fourth, the gear engages fine but the car needs to be severely babied with throttle for the car to accelerate. The car slightly vibrates too. There is no slipping as far as I can tell as my RPM's remain stationary. Car is unfortunately at 280k clicks and I have no service history on the vehicle so I'm weary to change the fluid. Wondering if this is even trans related and maybe it's a misfire but it's odd that it's only in fourth while third and fifth are fine. Any ideas or similar experiences gents?

If I recall, 4th in a 5 speed would be a complete 1:1 ratio, so the engine/transmission speeds are matched. 5th is overdrive...that's odd that 4th is causing problems. I would lean towards a gear issue, especially at 280k. At most, you could drain a little fluid, check the drain plug and fluid for metal fibers/fragments, and replace with the exact amount of fresh fluid, and see if it frees it up a bit. It could be a sticking valve in the transmission, but I'd probably look towards fully servicing it. A misfire should normally trigger a CEL and store a code.