Gambit should NOT be Logan's replacement. I think they should both be in X4. Their similarities should be acknowledged, but they should be established as two separate characters.
Plus Hugh is almost forty... this guy isn't supposed to age no?
If his age is too obvious, they can de-age in X4 like somebody else said. Also, they might be able to shift him into the 4 Horsemen in X5, where the characters aren't supposed to look so pretty.
Not sure about that last option, since he was the centerpiece of the first trilogy. It may be hard to make him a villian two movies later, but it's definitely an option that should be explored.