I know this is not the main subject in your thread (or even this forum :S), but isn't a 4h schedule too much for a baby this tiny? In Tracy's books she recommends starting with 3h interval between feeds for most babies, 2,5h for preemies and small babies. Maybe that's making naps harder? He may be hungry or OT when you try putting him down. Just a thought....
About your question, when I go out with DD (8 weeks old) I usually try to do it after a good nap and a feed, so we have some A until she falls asleep in the pram or the car seat. When she wakes up, I feed her, and try to be back at home before she falls asleep again (so she doesn't have 2 naps in role out of her crib).
Congratulations on your new baby!
E.