I started on a Trek 4500 a couple years ago and beat the crap out of it and it held up great--and I'm no small guy at 5'10" and 230lbs. It's a great bike for the money. Plus, with a lifetime warranty on the frame, how can you go wrong? Sure the components aren't top notch, but what do you expect for $400. Just ride it until you feel your skills require an upgrade, then save your $$ and go FS. I think that's what most people do and it makes sense. I upgraded to FS after a year on the 4500, but I still keep it around.
Now, 2 years later, add some extra cash and boredom and my '02 4500 is super upgraded. The only stock part left is the frame. I don't recommend dumping upgrade money into it from the start, instead save your cash for a better bike. I'm just saying that with a lifetime frame warranty, why not keep it around for awhile. It's a good sturdy frame. Plus, back when I bought mine, almost all the Trek HT frames were the same material. You spent your $$ wisely, now enjoy the bike and keep it around for awhile.
As for the difference between the 4300 and the Spec Hardrock Sport, I think they are pretty equal. I don't think you could go wrong with either bike. Don't rule out other makes and models though. Test ride everything in your price range and make your decision based on that. Try not to go into the shop set on a certain bike without riding it first. It's good to have an idea of what you want, but remember to keep an open mind. Buy whichever bike feels the best. A bike that you enjoy riding will keep you riding it. Nothing kills your motivation like having to ride a bike your not comfortable on.