It is my understanding that Tohatsu manufactures the portable four stroke Mercury and Evinrude motors and of course, they sell under their own name plate as well. I bought a new 2008 - 6 hp Merc four stoke to push my 17 ft aluminum boat. I found it to be a good starter and it ran well at medium and high RPM's, I found that it ran rough at low RPM's. It was also louder than I was hoping for at higher RPM's. It pushed my boat at a maximum of 6 Mph and it did have trouble with maintaining a good constant speed below 3 Mph. I found out that anything less than an 8 HP is a one cylinder and starting at 8 HP they shift over to two cylinder. It bugged me enough that I changed to a 2013 9.9 Big Foot and it is way smoother at low RPMs, runs quieter and it idles down to a lower speed and holds the lower speed much better. It pushes the boat at about 8 Mph on wide open. My advice would be to spend a little extra to get an 8 or 9.9 HP (9.8 if you want Evinrude but it is the same motor and it is quite a bit more money).
Fuel economy is awesome with the portable four strokes and I would highly recommend to go four stroke for fuel economy reasons. I really like the better speed control that you get with a portable motor as opposed to the big motor (mine is a 90 HP) but I would have a portable for safety reasons only. Over the years of having boats I have had to troll back to port at least four times because the big motor failed. It happened once with my new boat and I trolled back to Burwell with the 6 HP - I found it later that the problem with the big motor was just an air bubble and I would have been able to get it going but the portable motor was handy and I picked up a bunch of wally's on the way back in as a bonus!
I should mention that the 9.9 Big Foot is a little more heavy duty than the standard 9.9. The more rugged Big Foot is only available with the Mercury nameplate (as far as I know) but I am quite pleased with it.
My final comment has to do with motor break in. The four strokes need to be properly broken in and while this takes some time, it is an important process. If you are buying a used four stroke, be certain to ask the seller if they followed the break in procedures. If they look at you like they don't know what you are talking about or if you have any sense that they did not follow the break in procedure, I would not buy that motor.
JB (Wood Duck on radio!)