Air, that was a huge concern when designing goblins. It is not meant to work that way, and certainly, if you were to play goblins only to get to maximun city size faster than the rest, to then migrate them to a more powerful race (with the implication that you could also "breed" a whole lot of cities that way), you would be abusing them. Not that it would surprise me coming from you anyway...
Why not suggest ways to make the goblin high level units more useful ?
As far as lvl4s go, I see 2 problems:
1) The recruitable lvl4s are obtained relatively easy now(either by some poorly guarded dwelling, or some quest reward). They provide excellent opportunities to expand quickly and with minimal casualties. You could even rush with them. The high price is no object either. In allied games, allies can provide the money, and in FFA's, I think they pay by themselves with the indies they can take on their own. Stop building at your cap to get them, is usually OK.
2) The buildable lvl4s are, IMO, not worth going for. Their very high price (that translates into very long building times), plus their relative weakness vs other units, incl. heroes (and I'm not talking about retaliation limitations here), make the effort (and risk), not justified in most cases. The late game fase when they are ready to come is also a factor because when the game is about to be finished, what difference can they make?. As far as my experience with high level units go, not many units above $200 are worth the trouble.
But I understand they are the less experimented units, and not much can be said about them when you rarely have the chance to test them.