If you can finance at 4%, then after you've started the job, that is a rate worth taking the financing. I haven't seen many 4% car loan offers lately though. At a rate higher than 5%, I'd recommend just paying for the car out of savings. (Mathematically, an 8% speculative return isn't worth as much as a 5% guaranteed return.)

You may need to finance the car for 1 payment period because a lot of new car incentives are through the dealer financing and not the dealership directly, but just ensure that there is no prepayment penalty and you can pay the car off when you get the first statement.

Edit: Also, unrelated to your question, make sure you have a 6 month emergency fund in a savings account beyond what you are spending on this car since you say most of your savings is in the stock market.