400k+ in two days is pretty damn good for a game Nintendo basically threw together in 10 months with a B-team.
They're gong to sell through the first shipment and several more shipments after that no problem, lol.
I'm actually glad it might "only" sell 3-4 million copies in Japan LTD ... if it sold 6/7/8 million like NSMB DS, I think it would give Nintendo no incentive to ever really do anything new with 2D Mario again.
Nintendo will still make a ton of profit with this.