Here's a little chart - a bit "tongue in cheek" but appropriate.
Between the 30 and 40 horse 4 strokes you are looking at somewhat different motors.
I believe that the 30 is built by Tohatsu and dressed as a Merc (so essentially a Merc, Tohatsu or Nissan - take your pick).
The 40 horse is built by Merc at their plant in China along with the 50 and 60 horse (4 stroke) models.
So maybe toss the coin between Made in Japan or Made in China.
A 4 stroke does not have the low end "guts" you get with a 2 stroke - important if you want to pop up a skier (or something like that), maybe not important otherwise.
At "lower" rpms a 4 stroke will burn less gas than a 2 stroke (generally below 3000 rpms - but nothing written in stone).
At full throttle a 30 2 stroke and 30 4 stroke both burn about the same amount of gas and while they "sound different", most testing shows the actual DB readings are about the same.
So a 4 stroke is "quieter" and "more fuel efficient" if you never pass 1/2 throttle.
There is many times more moving parts in a 4 stroke compared to a 2 stroke - parts break down over time so your long term repair/maint costs will generally be greater with a 4 stroke.
4 strokes are "less forgiving" of routine maint schedules than 2 strokes. Grampa's 2 stroke can be neglected in the barn for years - a carb rebuild, new plugs and fresh gas and she's running like a top. Do the same with a 4 stroke and you have a paperweight.
There used to be a "significant" weight disparity between motors. The gap is closing but in general a 4 stroke is still "heavier" than an equivalent 2 stroke (as far as horses go). Hauling that "extra weight" all the time does cost you some gas.
So, if you generally run at lower rpms AND you are willing to treat the service schedule as gospel (instead of a simple recommendation) and don't mind the extra weight a 4 stroke is a viable option.
If you are somewhat a procrastinator when it comes to service an old carb model 2 stroke might be more your speed.
Alternately, in the horse class you are looking at you could opt for a DFI built by either Evinrude/BRP or Tohatsu - but a DFI is a very complex motor and usually requires the service to be done by a dealer - not something you work on with a screwdriver and multi-meter in the driveway - but still, in my opinion, the superior alternative to a new 4 stroke.
