I'm new to mountain biking, and I am looking for a bike.I am 6' 4"tall.LBS has a new 'o4 trek 4500 $100 off, in frame size 19.5". Would I be better off spending the extra money to get a 21" or 22.5" frame? ('05 models)

If you're looking at the 4500 i'm guessing you aren't looking to do aggressive stuff. So the bike doesn't need to be super small. If i were 6'4" i'd probabily go with the 21". But to be sure go down to the shop and try all three sizes out to see which is most comfortable.

Without knowing your inseam measurement, I'd only be guessing, but the 19.5 seems a bit small.

Do you have unusually short legs?

My inseam is 35". The bike shop doesn't have the 21" or 22.5" bikes in stock, so I can't them. The 19.5" was ok on the parking lot but I have long arms and thought maybe the extra length would be less tiring. I plan to ride some pretty steep mountain trails,but not at competition speeds.I am 48 years old, still in pretty good shape.(lots of running) I have ridden some trails on my old diamondback but it is way too small.

With a 35" inseam, I'd get "as big as you can". The 22.5" might be just what you need.