l have a 2003 Forester with automatic transmission.l noticed on the freeway today that while on cruise control, it would downshift to third gear when needed but would not upshift to fourth when it no longer needed extra power.

take it to an AAMCO transmission specialists. if there is something wrong expec$ to pay $1000 to $1500.

Oh boy!
If ur car is still under warranty.take it to the dealer. l had a problem with my 99 Forester not changing into Drive at all.from parked or reverse. Ends up l need a new transmission.paid out of pocket $ 1600.00.ouuu! But get it checked our because it maybe a specific part within the transmission that they can fix.cross ur fingers. And anyways ur car is rather new to have a transmission problem.but it all depends how you've treated the car & have done the routine checkups.

Check the cruise control and transmission fluid. When was the last tranny fluid flush?

Without driving it, l can not make an exact diagnosis. But..does it shift to 4th during normal driving? Can you manually upshift it to 4th while driving around town? If so, there is nothing wrong with the tranny. It may be a function of the speed you were driving and the need of the vehicle to be in third to sustain it.