I do think it will sell less than 4 and 3 did. Afterall 4 was cross-gen and 3 had killer sales thanks to it's high 91 meta. Hard to say how much less it will sell than them though. I think it'll top out around 9m lifetime, 4 and 3 both passed 10m.
I always find it hard to remember just how good cross-generational titles have sold.