Hi and welcome to COL
Don't let it become a habit as it's much more difficult to retrain a bad habit than to train a good one in the first place. Practice some recalls as you're walking round, sometimes putting him on lead for a few seconds, sometimes not, then
before you get to the place where he runs back to the car, recall him, put him on lead and keep him on lead until you get back to the car. You can practice walking on a loose lead and keep his focus on you rather than the car by doing frequent changes of direction and encouraging him to watch what you're doing. The recall practice is important anyway, regardless of your current problem, because in a few months he'll become an independent teenager and pretty much everything goes out of the window then.
Can you vary your walks so that it's less easy for him to pick his scent up and run back? Are there different places you can drive to or different routes round the place where you usually go? Dogs are quick to learn and to anticipate where you're going if they do the same route frequently. If you're stuck with the same route try walking it the other way round sometimes, or changing direction within the walk. It will all help him to focus on you, as will doing some training while you're out or playing games.
How long are you walking him for? At 4 months his walks should only be about 20 minutes as the general rule for puppies is 5 minutes walking per month of age. You can do 20 minutes a few times every day and the occasional longer walk won't hurt, but too much exercise before the growth plates close can lead to joint and ligament damage at an early age.
Finally, we'd all love to hear more about your boy and see some photos too please
