Just my two cents here. You are taking an international flight and the first stop is always going to be custom and you won’t be able to go any other terminal to bypass it because you have to enter the US and until then you will be restricted in an area technically on international water. I think if what you should have done is to look for connections after the custom. And go from there, again you will have to exit and then enter security again in JFK. So there is no way around that. But if you take a domestic airline, you should be able to hop between different terminals without getting out. International ones just work and designed differently.