Based on all of the information here, I would suggest doing a complete drain and fill of the fluid and replacement of the
internal transmission filter. (no gaurantee this will work however) Using synthetic trans fluid would be a good idea, but in this case, I would stick to a normal "cheap" trans fluid until you are sure that the transmission will last. (no sense wasting extra money) If you had similar symptoms related to a possible TPS before the transmission fluid leakage, then the TPS could be a possible culprit but it really sounds like the friction surfaces have received some extra heat and wear. No need to replace any solenoids at this point unless you decide on doing an overhaul of the current transmission.
4th gear is engaged by the brake band in addition to the high clutch with the brake band being the probable problem here. If it needs to be replaced, it would include pulling the transmission and a proper rebuild to be done.
I believe there are a few forum members in the NW that could assist or know of a good place to have work done. (including a 4.44 swap) As for the fluid exchange, I would stay away from jiffy lube or any such similar place and go to an actual garage, preferably one that knows their way around Subarus.
Edit: By the way, I just want to thank you for including such detailed facts pertaining to the situation as well as doing some research of your own. This really helps your chances of getting a proper diagnosis and help from the experienced members here.
