semi bluff out of position??

[KHALI]

3/6 Others are typical passive types and I didnt fear a raise form BB who was weak player. Hero is SB with Kd 8d

UTG limps, CO limps, Hero calls(debatable?), BB checks

Flop Ac, Qd, 4s

Hero checks, BBchecks, UTG checks, Co checks.

Turn is 9d

Hero Bets


What do you think of the semibluff here? What conditions would you make this a bet if you think it is a check/fold or check/call?
Any general rules you use when to bet and when not to? Heads up versus 3 handed? Number of cards above nine on board? ANy thoughts?