Well the 4500 isn't a bad bike in its price range, and its good to see it comes with a J2 fork, so i dont think that will be a priority upgrade (you could send it to the lbs for a service to improve performence if you wanted)..
If there is anything wrong with your bike at the moment compontent wise (broken or damaged)? i suggest replace them first. Upgrade wise I would recommend upgrading the brakes if needed (i'm assuming it has v-brakes?), if you dont think the brakes need upgrading then you could at least get new pads if the bikes a couple of years old, also the rear deraillure might be worth an upgrade if your doing a lot of climbing and changed terrain, precision shifting is always a plus (do you have a 9speed rear gears?).
To make your bike perform better you could also try a new chain and cassette, replace the deraillure and brakes cables to make the ride feel a bit smoother.
It depends though on what on your bike actually needs upgrading, and then it comes down to what parts are hasseling you and you would like to repair/replace them.