Under NFHS rules, does a player need to have all four points snapped up? Any rule reference would be wonderful.

Only the NCAA mandates a four point chin strap.

However, the NF rule says (1-5-1b) that the strap must be properly fastened. An NF interpret says this means all available points must be fastened.
I look at it this way, if the helmet has four points, the strap better have four snaps. If the helmet only has two points, then the strap should only have two snaps.

I don't even recall seeing a 2 point/snap helmet and chin strap in easily the last ten years, maybe longer

You know the kids see the NFL stars with one or more
straps flapping loose and copy them--- if I see any not
fastened, I tell them "snap 'em all up !" If they persist
on blowing it off-- I will flag them before the
snap. Hopefully that will get my message across.
btw---
Back in my college playing days we only had the
2 snaps on our Rydell suspension helmets---
remember them ??

The helmets have four points for a reason...safety. If all four points are not snapped, the helmet is not being worn in the safest possible manner.