I'm not sure if this sort of equation works with microwave ovens, but pro-rata it would work out at 650/900 x 4 = 2.89 minutes... I think I would check after 2mins, 30 seconds but allow 2mins, 50 seconds... heck, I'd just go for 3 minutes but check after two and a half minutes...
I did check every 30 secs, but it ended up being cooked for 4 mins and it was still a wee bit soggy in the middle - but it tasted luverly anyway (esp with the custard and ice cream)