Getting Check-Raised on River W/ Nut Flush Headsup

[DefThreats]

I am playing 5/10 headsup on Bodog against MypetTurtle, who I have not played before but seems ok so far. Stacks are effectively $1500.

I raise Ad Qd on the button to 35, Villain calls.

Flop: Ac 8d 4s
Villain bets 70, I raise to 200, Villain calls.

Turn 2d
Villain checks, I check.

River 4d
Villain checks, I bet 400, Villain raises all-in for roughly 900 more. [img]/images/graemlins/confused.gif[/img]


Thoughts on flop, turn and especially river?