I've been having a problem with my 98 honda civic. However, the problem doesn't seem to occur when the engine is cold, only after it has warmed up. It doesn't appear to shift down when I come to a stop. It seems like, when I try to move again, it's in a higher gear. I can feel slipping and a lack of power.I also have trouble hitting fourth gear when I'm on the highway.
Any help is appreciated.

Try changing the transmission fluid. See if Mobil 1 Synthetic Multi-Vehicle ATF, available at WalMart, meets the specs for your Civic. ATXs downshift and upshift much better with synthetics, besides a host of other benefits.