More likely they will go for M4s not SIGs!!
If by M4 you mean AR-15 pattern rifles from Colt, may I ask why?
End of the day all modern AR-15 based platforms from reputable US companies are very similar in functionality so it really doesn't matter. Though must say the SIG 516 which incorporates all the good things from the HK416 design is a pretty compelling case.
But what's worrying me is the loss of P90 its a really good rifle. But we have other alternatives in the form of Israeli weapons.
I wouldn't call P90 a rifle, more of a PDW.
Again, it was a solution for Close-quarter situations created at a time when effective, short-barrel AR-15s did not exist. Today that's not the case so its very hard for me to recommend a P90 over something like a SIG 516PDW with 7.5" if you want a piston or a Daniel Defense Mk.18 if you can do without a piston.
Unless perhaps in very special use cases like VIP security details which may sometimes necessitate the very rectangular-shaped P90 both for ease of concealment inside cases or maneuvering in very tight spaces. But its certainly not indispensable anywhere else - there's a reason why no serious Military SOF uses P90s in the world.