So the first question is, why did it SIT for four years? Why was it not used? And is the answer given to the buyer truthful?
I have been burned many, many times over the years from untruthful sellers. I knew it every time too but luckily I was able to fix my mess up and recover my losses on all but one occasion.
anyway, if efi it still should have a drain on the vst (vapor separator tank)--which is a small tank mounted on the side of the motor somewhere. I ain't never seen one that didn't have some sort of drain on it. If it's got anything left in it check it-is it really nasty brown/green stuff? If so, you can figure on pulling the vst apart and replacing the fuel pump as well as the injectors--just to start. Possibly fuel pressure regulator. And probably the low side fuel pump. Check the lines too.
if carb drain them. Or at the very least one of them and if it comes out brown/green/stinky, figure on a carb cleaning/rebuild. Some of these carbs can be a real bear too so you may find it beneficial to pick up a service manual. When doing the carbs, also do the fuel pump. Check the lines too, bend them pretty tight and if they crack, replace them all. Nothing worse than being out in the middle of the lake with a huge fuel leak and/or a non-running engine. And usually that happens when nobody else is around to help or tow....
water pump for sure. Might as well just take the foot off before you ever try to crank the engine over. A lot of times the impeller will just fall apart and starting the engine will force the pieces of the busted impeller up into the water tube and wherever else. So if I know a motor's been sitting a while I usually just take the foot off before turning it over.
Also on 4 strokes, timing belt. Many of them have a timing belt system and obviously belts are usually rubber, they too can rot.
Around here, doesn't matter if it's 2 or 4 stroke, when they sit the dirt dobbers will build nests under the cowling. Most cowlings will have a removable silencer, take it off and clean it. If the motor has signs of dirt nests on it, clean them off immediately. Outboards don't have air filters (some have a "filter" but it doesn't really filter, it's just a silencer) so any dirt that gets under the cowling goes right into the engine. I've seen tons of destroyed powerheads due to mud dobbers.