"Robert, depending on the year
"Robert, depending on the year, it looks like you may just need to "re-jet" the carb. If that is all that is required, then well worth it (along with the appropriate carb adjustments). Otherwise, if it requires a new carb (200 bucks or more) then it is probably not worth it.
The on boat performace difference between a 4 and 6 horse would be minimal at best...
I am however using a Merc service manual to determine this - however, since the mid 1990's the little Mercs have been built by Nissan/Tohatsu. In fact the only difference is the colour and decals."