I think rather or not you move up, either size in a new 4s would be more fuel efficient than the 2s.
The question is, do you really need or want to spend the extra money,when it's mostly a kicker motor and 4-5 knots is good enough to motor when needed.
I had a 5hp Merc on a 26' that motored quit well if needed ( 4-5 knots ),.
1978 Tayana 37
1951 Blanchard 51 Custom ( I got a woody )