I suspect you'll introduce a gap. The 4H will probably launch higher than your 3I, though, and might effectively replace the 3-Iron. Is there a 5H in that set? Maybe get a 4H and 5H to replace the 3 and 4 irons (test this on a launch monitor before buying). If you're really worried, see if you can get the 3H cover to put on the 4H, and then put your 4H cover on the 5. You might start thinking of them as 3 and 4 despite what's printed on them.
Whenever I hear someone hit a shot with a particular TaylorMade iron/hybrid, I always add one to the number. Hit something with your TM 5-iron? I think of it as a 6. And so on. I hope 19-degrees doesn't end up as the new 3-iron.
(Sorry; I'm too young to be this cranky about changes in golf, such as lofts)