Actually, the fourth ship is totally worthless. I don't know if it becomes interesting later when upgraded, but it's clearly a bad ship. Why ? Because it's far too slow, its damages are equal of the second ship, which is destroying targets much quicker than it. It's sad.
Options I thought about were : or you increase the fourth ship's damages by a LOT (x2.5-x4) and keep the same fire rate, or you increase its damages (x1.5-x2) AND its fire rate (x1.2-x1.5). But right as it is, it's clearly not worth the expense of the upgrades, and especially not worth the expense in energy in order to summon any of them. :s
PS : Farming the same level again and again and again in order to get some low rewards QUICKLY becomes annoying. I've no good idea to fix that, though.