I contacted VMC as to the difference between their 4X and 6X rated hooks and here's the scoop:
4X strong is 4 times stronger than standard wire diameter and 6X is 6 times stronger than standard wire diameter.
The 4X is high carbon steel XC 80.
The 6X is a little shorter shank and uses Vanadium steel that is lighter than regular high carbon steel...you will be able to replace the 4X with 6X without killing the action on any lures. I would be more careful when it comes to spoons as they are weight sensitive.

I dunno, size for size they feel heavier to me. Maybe they just look heavier so it makes me think they are. I don't think I've ever put them on a scale though.

Heavier..