The most common issue will be a stripped 4th gear clutch hub.
For the other gears to operate properly, the soleniods can not be bad. There is no valve in the trans that I am aware of that can stick to cause 4th to not work.
There are a couple options available:
1. Lower your cradle and engine/trans. Remove the side cover, then pump/valve body/channel plate as one unit gaining access to the gears and shaft. Replace shaft and channel plate gaskets, reassemble.
2. Have a trans shop pull and rebuild
3. Get a junkyard trans and swap. The chance with this option is that you get another trans with a bad 4th hub. Sawgunner recently went this route and was unlucky.
00 Regal GS: All kinds of cool stuff.