To reach those 3-4M then Switch may not outsell the PS4 by more than 1-2M through October, and sell on par in November, as the 2M gap in December will certainly happen again.
I seriously doubt that to be the case, even with a pricecut for the PS4 and no pricecut or revision for the Switch.
Switch is selling without any releases on par with the PS4 with big releases (like, The Division 2 this week for instance). This can go on until Super Mario Maker 2, where Nintendo starts shifting into higher gear this year. Come summer, and the slew of Switch releases and nothing big on PS4 side will make the gap grow pretty fast during summer and fall, before the PS4 getting trashed by the Switch during the holidays.
Sorry but a sequel of The Division isn't a big game or something that pushes HW up for any notable amount. Smash and Let's go tail push more HW than it. I would say that Oddissey and Zelda as evergreen they seem to be also push more than The Division 2.
duduspace11 "Well, since we are estimating costs, Pokemon Red/Blue did cost Nintendo about $50m to make back in 1996"
Mr Puggsly: "Hehe, I said good profit. You said big profit. Frankly, not losing money is what I meant by good. Don't get hung up on semantics"