Ok well i have been looking at trek bikes for a while and decided its what i want. I have been looking at new and used bikes. I was looking at buying a new bike but then i found a 04 trek 4100 for $100 which beats any new price. So should i buy that put about $200 into it and save myself $100 or is it better just to have a new stock 4300?

Just get a new stock 4300

Why would you need/want to put $200 into the '04 Trek 4100?

If there is something amiss and you need to do it, then go with the 4300.

If there isn't anything wrong with it and you want to upgrade $200 worth of stuff, then still go with the 4300.

If there is nothing wrong with the '04 and it works fine, I would say go with whichever one is sold from an LBS. I wouldn't buy a used bike from anyone that isn't a respected member of the cycling community. If the seller is something of that stature, by all means, save yourself $300. But, of you are just saving $100, go the the 4300.

I would go with the 4300. I like that model and have sold quite a few of them (if you haven't guessed, I work at a bike shop). the 4100 would probably do fine but unless it was checked over by actually bike mechanics, I would guess that stuff would go wrong.

I love my 4300, it's a great bang for the buck. I do moderate trail riding, 2 years of beating the **** out of it, no problem.