Well you can call that a shot
Even if it do over performing sales with combined Jan-Mar having over 1 million... you still needs 2.7m in December.
That would be 4 months, but I get your point :p
2.5M December and 300k a month for Jan-Apr would do it. Like I said though it'd be crazy if it happens. The post-April comparison favours PS4 anyway, so there's no need for Switch to be ahead. If its ahead at the end of March then its still ahead when they're placed on roughly equal footing. Last March there was only 100k between them launch aligned.
Edit: well here's Switch's totals for those months a year ago
Dec: 1.5M (any more accurate numbers out there?)
So 300k a month for those four months doesn't seem too unreachable this year, April just needs to be much better. I could see something like 2.3M in December and then 350k a month for Jan- April being the roughly most likely route, if it does happen. 2D Mario should help with the earlier months, and then if Animal Crossing makes April it should be all good. My current prediction is something like 1.9M Dec and then 300k average over those four months for total 3.1M, falling 600k short of PS4 launch aligned.