D4 does jail into F344; D4 is +17, while F3 is 14 frames. It requires practice, but you can indeed hit your opponent with your F3 after D4 on hit before they can duck. If you missed it, that means you were a bit too slow, which occurs frequently because Cage only has a 3-frame window to jail it.
Honestly I think you are giving the F3 and F4 a bit more credit than they deserve. They aren't very fast so it's very easy to tell what Johnny is doing once he chooses and since everything he has is either high or Low for the most part you can pretty much ALWAYS duck low and it will be the right call. So after they get hit with the D4 they can just always duck and wait to see what his animation looks like and react. especially with the F4 because it's very obvious if the opponent knows Johnny well. Which means for either the F3 or Grab all they have to do is duck without blocking and then choose to block if they see his wind up for F4.
I'm not saying that JC players should always do F3/F4 after a D4 on hit; we need to mix up our follow ups. We can jump in after a D4, we can F212, we can throw a fireball, we can step back and not attack at all and just react to what the opponent does. My point is that Johnny definitely has mix-up options to apply to the opponent; we just have to be creative with their application given our lack of Scorpion F3-tier mids.
I understand that there are many options to choose from but my point is that johnny is a character that needs to be aggressive a majority of the time to function properly and a lot of the options aren't great against someone who knows johnny. Sure you don't have to attack but you give your opponent time to back away then and a good player will blow you up from that distance when you jump by doing a standing 1.
I'm not quite sure that Cage was designed to be aggressive though. As you noted, some of his best buttons are on the slower side; I wouldn't call 14 frames slow, but compared to 9/10/11-frame mids, his could be considered slow. His B3 is 18 frames, F2 is an 11-frame high; Johnny Cage strikes me as a whiff-punishing, mid-screen footsie monster. Ms. Briggs needs to be aggressive, as she is this game's version of MK9 Johnny Cage.
I honestly don't feel like he has the best tools to be mainly a whiff punisher, if this is the case. I honestly think he isn't bad but he is very flawed as either a rush down character or a whiff punisher, especially against a lot of characters. I feel like the way he is designed is a bit awkward if he is supposed to be either of these playstyles. Again this is only against someone who knows him well but that is an issue for me, especially since I love Johnny. Also you can still standing 1 people who are doing jik's as well if you time it right.
^ THISHop on the train now so that when he gets his buffs, no one can call you a tier whore
Seriously, though, he rewards strong fundamentals for sure. He may not have the same insane rushdown as he did in 9 and X, but he still has ways to keep the opponent guessing as long as you don't auto pilot.
It takes getting used to, especially if you're coming in with that rushdown mindset, but I can't think of a character more satisfying to win with, once you get it down.
) and the f34 is impossible to land in a match. But damn he is so fun to play. Fix him and he would be awesome.
he has overtaken liu kang as my main until sindel arrives. 
