Depending on the age of the car you can still likely get a car loan for a private party sale. Have you actually applied or talked to a banker about a personal loan? The credit hit is minor and doesn't matter if you are not planning on using credit in the next few months.

I dislike 401k loans in general as you are taking money out of the market, and depending on the plan rules may need to stop contribution, might not be able to pay off early, you would have to pay it all off should you be fired or leave before it's paid off, etc. It's also just bad practice to think of that 401k as a place to go get money from when in need. This is a want, not a need, not an emergency.

This is the type of thing savings are for especially when you can replenish in such a short time frame.