I'm getting ready to build a 7'6" H&H S-glass 4wt I was thinking single foot wires but wondered if the double foot snake would slightly stiffen the blank. I'm trying to keep it as light as possible. Any thoughts.

It will not Stiffen the blank. Any additional weight added to a blank will "dampen" the recovery. Think of it this way, Diving Board for a swimming pool, then strap a concrete block to the underside, it will dampen the recovery of the diving board.

Depending on brand of guides, there is very little weight difference between single foot and double foot guides. Personal preference. I like the traditional look of a Snake guide and I think single foots look awful on a fly rod, just my 2 cents.

i can,t remember the last time i wrapped a double foot guide of any kind..using a locking wrap on single foots made double footed guides obsolete to me..

My 2-cents is that single foot advantage is basically 1/2 the wraps. Individual preference if you think they look good or not on a fly rod. Virtally all OEM Euro Nymph rods are now using single foot. I can't say I see a special need on a nymph rod for single foot. My last rod was the first I did with single foot guides. It looks fine to me, granted less traditional. I guess one could make the argument that a single foot might pull out easier if a loop, knot or transition in your line/leader got caught on it when fighting a fish, or on a higher weight rod when ripping out a cast with a heavy streamer, nymph or spey line.

David, would it be a problem to use a locking wrap or some variation on the single foots used for nymphing rods...

It would not be a problem at all.

I absolutely prefer locking wraps as well. Not that hard and for a little piece of mind as well.