Leash walking just takes time and consistency. At 4 months old, I wouldn't expect him to resist leaves and stuff. Those are very interesting to puppies. As he gets older, those things will become less interesting. But you can take this as an opportunity for some training.
Use the leaves as a reward for focusing on you. A leaf blows by, say his name or a "watch" cue or even just "pup-pup-pup!" in a happy voice, and when he looks at you allow him to chase the leaf (I use a verbal cue "get it"). You will need to work on a "watch" cue in non-distracting environments first. Eventually you can make the game random and sometimes allow him to chase the leaves, and sometimes not. It seems silly, but by allowing him to chase the items on cue, you become part of the game and he will become used to looking at you for permission when he wants something.
Also work on "leave it". Again, start in a non-distracting environment first and gradually work up to distractions.
Ryker: (USDAA) SR (DOCNA) C-BTA C-ISLA
Solstice: (AKC) MX MXJ (USDAA) PDCH PKCH PJCH PGM PSM PRM (DOCNA) MEX C-IAA C-IGA C-IJA C-ISA C-ISLA C-ITA C-BAA S-BSLA S-BJA S-BTA