Im not sure I understand the question you are asking, but it sounds like you are asking:
"Can I put a 40k down payment, take a loan for the rest of the money owed on the car, then in october pay off that loan?" and the answer is already given but yes, in general car loans from financial institutions do not have pre payment penalties.
The only thing that may confuse you is, you will not be telling them " Oh by the way, I need a loan only for 3-4 months". You will just be taking a loan for whatever term you want to sign up for that they offer (3,4,5 years whatever), and then, deciding to pay it off in October.
You just need to verify that there is no prepayment penalty on the loan. Consider taking a loan with whatever financial institution you bank with normally, since the interest rate kind of wont matter for a loan you will only have for a few months, and having it at your regular bank will be easier to pay it off, if thats what you intend to do.