You can but a 2 stroke is simpler, easier to maintain, lighter, picks up better, not fussed about being layed down, much easier to sell on and holds it value far better(respected brand 9.8/15 hp 2 strokes have probably outperformed most other mainstream investments over the past 10 years). Have you read the sticky?
I've always put any unused pre-mix in the lawn mower, the oil won't do it any harm whatsover, in fact before we went all diesel it went in our old Focus - no problem!
Only disadvantage I can think of with 2 stroke is a bit more noise compared to 4 stroke and maybe the chance of oiling up.
If I was you I'd make a decision to go displacement or planing - if disp' then buy a nice little looked after 3.4 Zodiac airfloor and pair it with a 6 or 8hp, for this sort of use the 2 stroke maybe has less advantages.
Forget hard floors if you want to be able to lift it in and out of the canal/river/shore by hand.
If planing then a 15 2 stroke is the one. You can always put a hard plastic/wood floor on the airfloor - many do this.