All good points, and we may end up with the 3.5 too for the reason of weight. But it's good to note that the 4HP is the same CC as the 5HP and 6HP (the weight of the 5 and 6 is actually a couple pounds less because no internal tank). So you might as well get a 6HP if you're going to the 4HP weight and your boat can accomodate it. Of course it costs more, so that's easy to say.
I like that Honda 27 lb weight too. We lifted one up at the Boat Show and it really is light! But I worry it's too small to push a 10' inflatable in a chop.