With a $4500 budget, I would look for a used 06-07 KTM 250 xcw. With the leftover money, add a trials tire to the rear and buy up your protective equipment.
For rocky, super tight woods, over-heating could be a problem for an off-road noob with a 4t.
For those recommending a KDX......... while it is a fine machine for a beginner, someone that is as physically fit as the OP and has been riding mtn bikes, the OP would outgrow a KDX quickly. I would imagine the offroad learning curve is not going to be very steep and with your fitness, you could easily be a good C-class competitor in your 1st year.
all around good guy