Mach4 is nothing in comparision to mach 15 plus of ballistic missile ...
A ballistic missile does not attain this level of speed in boost phase; not even close.
Boast phase window is too short to take any real difference ... hardly a minute to react and that too if you are within 50 kms range ... in real life scenario chances of this is not even 1% unless US has complete air superiority in Pakistani air space and have hundreds of planes always available in air in Pakistan ...
More than a minute a actually, and it can be defeated in boost phase. Of-course, proximity is "vital." And yes, numbers are necessary as well. However, continue to read below.
So nothing realistic ... f35 anti ballistic missile capability can take effect only after getting complete air superiority but that will be tok late because if it comes to even close to this it means nuclear missiles are already launched ...
F-35 is designed to sneak through and/or jam modern set of defenses; no need to establish air superiority over Pakistani airspace beforehand, but whatever will come in the way of these Jets will be shot down anyways. Of-course,
numbers will be important for ABM missions. And you are assuming that PAF and IAF are not done with each other first (we are considering Pakistan versus India scenario, right?)
Nuclear weapons are last resort options, and I do not think that Pakistan and India will use them early on. Therefore, US will have window to intervene.
I see the possibility of US discouraging both Pakistan and India from using nuclear weapons in a war, and consider ceasefire. Towards this end, US might deploy its own forces in the Arabian Sea and threaten to unleash its F-35 squadrons to intervene and shoot down ballistic missiles from either side.
Now keep in mind that US already have extraordinary level of INTEL at its disposal, and Pentagon is most likely aware where Pakistani and Indian TEL are deployed. Therefore, F-35 can be dispatched to these locations directly, and their is no need to blindly patrol the airspaces of both countries. And F-35 series aircraft's sensor suite is excellent at illuminating military hardware on the surface for its pilot.
My point-of-contention is in regards to
hinted ABM capability of F-35 (technically speaking), and this is most likely true.
However, US might not bother with Pakistan versus India scenario at all, and allow the two countries to ruin each other.