Transmission slips in fourth gear around 50-60 MPH?

When I drive the vehicle on a level road, everything works good, shifting is good, but after reaching 60-65 MPH the gear shifts down and the RPMs go to 3+ from and then if I take my foot off the gas and gradually accelerate, it shifts into 4th gear, goes smooth up to 80 MPH in the last gear without any problem, and randomly downshifts only at 70+ MPH speeds and again, if I take my foot off the gas pedal and accelerate gradually it stays good in 4th gear. But when I'm driving uphill this happens at around 45 - 55 mph speeds, and if I do the same thing take my foot off the gas and accelerate gradually it will now stay in third gear as I'm driving uphill needs more power.

This is an used car I've had it for almost an year. Is this because of low transmission fluid maybe? I haven't changed transmission fluid, and I don't know whether the previous owner did it.