opts for different types of missiles on the S-400/S-500 rather than depending only on active/semi-active radar homing missiles like the 40N6,48N6DM, 9M96E2 among other missiles used by the S-400. So there can be a mix of radar guided(active or semi active) and IIR guided missiles.
There are at least 2 different missiles for the S-400 system and that does not include the two smaller 9M96 missiles that are part of the S-350 family. the S-500 will have something different too.
Regarding IIR guided missiles I really don't think integrating a guidance seeker that might see targets 20-30km away at best would be much use for the S-500 as the S-500 will be travelling at a rather high speed and will be intercepting targets travelling at up to 7km/s... even if the target is glowing brightly the 1-2 seconds an IIR sensor could detect such a target would not be that significant... it would be far more useful to have ground based sensors tracking the missile and the target and sensors on the missile also tracking the target.
Integration of sensors is normally a good thing and I agree it makes sense, but I suspect long range AAMs will benefit more than large heavy long range SAMs would.
Back in the 70s,Soviet operational analysis indicated that the low kill probability of missile seekers and airframes, especially if degraded by countermeasures, would be a major impediment to success. Therefore, Soviet Air Force pilots were instructed to launch two rounds, a semi-active radar homing weapon and a heat seeking missile.In this fashion the aircraft being targeted has a difficult problem as it must jam, decoy and/or outmanoeuvre three or four tightly spaced inbound missiles.
Improvements in technology and ECCM capability have reached a point where only occasionally multiple missiles would be needed... doing it as standard made sense when the missiles had less than 10% kill probability against long range targets, but now it is more like 70-80%.
the main use of radar and IR guided missiles was each complimented the other. A closing target is easy to track and guide a SARH missile at, but the front of an aircraft offers a poor IR target.
Conversely a receeding aircraft is hard to track with a pulse dopplar radar and the engagement range of a SARH missile for a receeding target is dramatically reduced, yet an IR guided missile looking at the engine exhaust tubes can get hits at much greater ranges especially with low flying aircraft with heavy loads in full AB.
It is not an accident that the short range early MiG-29s could carry two SARH R-27s for short range air defence while the larger Su-27s which might not be engaging targets head on also carried the IR guided R-27s.
If I was going to guess one of the secrets about S-400 is that it's a transitional system between anti-IRBM ABM's such as S-300V4, and anti-ICBM ABM's such as S-500 and Moscow ABM.
S-400 is a replacement for the S-300 and is not in the same defence structure as the S-300V4 which is an army system.
Likely it will be Verba, Morfei, Pantsir-SM, S-350, S-400, S-500, Moscow ABM for the air defence forces and air force.
For the Army it will be Verba, Morfei, Tor and SOSNA-R, Pantsir-SM, BUK-M3, S-300V4 and possibly S-500.