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
Freedom comes when youíre ready to sail away. True freedom comes when you donít have to return
Cut off from the land that bore us, betrayed by the land we find, where the brightest have gone before us and the dullest remain behind, .......but stand to your glasses, steady,.......tis all we have left to prize, raise a cup to the dead already, hurrah for the next that dies