It isn't the safest thing to do but I'd probably let my pup walk before the rabies hot.
Waiting 6 months for a walk is just so long in my opinion. I would not go anywhere on public ground with pup until 1-2 weeks after the last set of distemper, parvo and hepatitis for sure. If your state law says rabies by 4 months and your vet is makng an exception, it may be best just to hold off on the walk though.