OK. I can't help myself; I have to wiegh in with some of my own theories. I agree that on its face, this doesn't sound like a good idea, and you're likely going to break your rod. But If you're willing to take the chance, more power to you, and a broken rod could get you out from under some of the ethical issue involved (or at least provide some penance).
As I see it, using a 4wt for coho brings up questions re two types of ethical issue: conservation ethics, and sporting ethics.
If you're serious about catch and release as a conservation tool, using a 4wt for coho is clearly unethical. I'm sorry about that, but it's just too likely that you'll kill most of the fish you play, if you can somehow successfully land them. And don't feel better just because they swim away; those fish will not survive to spawn. Now, maybe you'll get lucky and hook only hatchery fish, or maybe you don't care about conservation, which is your right. But you should be aware that if you do kill a wild fish, even unintentionally, you could be liable for a big fine.
As for the sporting issue, using a 4wt for coho could be considered an interesting challenge, well within the traditionally acceptable "conceipts" often employed in angling, particularly flyfishing. After all, if we didn't do it partly as a "frivolous" test of ourselves, we could just use a net, or rotenone.
However, to be truly ethical, I think you'd have to take it all the way. One of the "correct" functions of a 4wt is to protect fine tippets. As you know, a 4wt WILL land a big fish, and in some trout fishing situations, it may be the ONLY WAY to land a big fish, because you need the light rod to protect the 7x tippit it took to get that big fish to rise. So to make using a 4wt on coho truly "sporting," you should use the fine tippit that the rod is designed for (not necessarily 7x, but maybe 5?), which will force you to play the fish with even more care. (This is what line-class records are about.) Otherwise, the whole thing is just an exercise in making the rod bend more.
Of course, this makes it even more unethical from a conservation standpoint, as you're even more likely to kill the fish. On the other hand, it has a better chance of escaping, possibly early on, before it becomes fatally exhausted.
Boy, it's like a conundrum inside a riddle (presented by a windbag:AA ).
PS: Please know that I don't mean to imply any value judgement; do what you want, as long as it's legal. I just like to go on.