If S-400 is unable to identify if the incoming missile is a MIRV or not, it is definitely not able to identify if it's a cheap empty missile with only fuel in it..
Pakistan should develop 100,000 dollar worth of fake missiles in large quantity. If the war starts, Pakistan should first send these cheap fake missiles (10s of them even 100s of them if possible) in the territories where Indian S-400 systems are placed.. in a couple of days or three days, this tactic will exhaust all the S-400 ABMs in Indian inventory... once it is ensured, Pakistan should start using the real ones.. on air bases, army headquarters, airports, major banking headquarters, industries, power plants, etc etc etc..