The 2004 4EAT can use a Dexron III-type fluid. The HP fluid is backward compatible, and can be used in all 4 and 5 speed automatics, but it's somewhat more expensive. (5-speeds require HP. There's no confirmed equivalent available in North America that I'm aware of.)
I'd probably not want to mix fluids, so if the Dexron III is in there, and given that draining the pan only removes a part of the fluid, perhaps stay with it.
Many threads here report improvement in transmission operation after a multi-fluid change, with or without the spin-on filter being changed. (See the "Changing . . ." linked thread above.)
There's really no need to remove the pan and change the pump pick-up screen unless it's suspected to be blocked for some reason.
Start by draining the pan and check the fluid that comes out for any distinct pieces. Also, check inside the drain hole for chunks on the bottom of the pan. If there's nothing, then I wouldn't worry about the screen.
The spin-on filter is also somewhat optional, but it's relatively easy. Use a genuine Subaru AT filter. As far as I know, no other aftermarket spin-on is built the same. (See:
http://www.subaruoutback.org/forums...ission-filter-different.html?highlight=filter)
If the ATF hasn't been changed in a long time, or ever, then changing it is the first thing to do.