No doubt this is going to be the biggest God of War yet, assuming it's going to be good. Not only has the E3 trailer got a crazy amount of views but the number of likes also massively outnumber Uncharted, Horizon, Zelda etc.
However, 4-5m FW is just unattainable, the best selling GoW(3) sold 1.5m FW and somewhere around 6-6.5m LT on PS3. Realistically GoW PS4 could sell 2.5-3m FW and around 8-10m lifetime (depending on bundling).