Flat calling AK for my stack

[ahnuld]

I played this hand a few days ago and didnt like my play on almost streets, particularly the flop. Was wondering what the better flop line is or if I did in fact play it correctly.

Tightish player opens for 20. I flat call with AK suited. We both have 400 and take the flop heads up.

Flop comes AJ4 suits not important. 44 in pot. He bets 40, I call. Turn is 8.

Bets 100, I call. River pairs the 4. Board reads AJ844. He bets 240 into 324 pot. I call.