Question: The sail is 4" longer than stock. It has been a while since I've hoisted my 17 main and I am wondering if that additional 4" on this non-Hobie main will be a problem for getting proper downhaul, especially for de-powering during higher wind conditioins.
It depends. Newer (since ~2000) Hobie sails are longer on the luff than older sails. On my boat (2007), I can nearly two-block the downhaul - even with micro blocks.
The only way to find out for sure is to put it on the mast.