Use your down payment $4000 to buy a used WR450F. With some patience you should be able to find a clean low hour '08 or 09 for that price. Dont finance a bike.
If you want to build your credit get a visa/mastercard (have your Dad co-sign if willing) and use it each month and pay it off in full each month. That is how you begin to build a decent credit score and credit history. You could even add a couple of gasoline credit cards or department store credit cards.....but pay them off in full each month.