Application of the overbet?

[Isura]

Villain is 30/4 and passive postflop. How much should X be?

Party Poker No-Limit Hold'em, $ BB (6 max, 5 handed) converter

saw flop|<font color="#C00000">saw showdown</font>

<font color="#C00000">Hero ($120.60)</font>
SB ($115.35)
BB ($69)
<font color="#C00000">UTG ($202.05)</font>
MP ($94.25)

Preflop: Hero is Button with 6[img]/images/graemlins/club.gif[/img], 5[img]/images/graemlins/club.gif[/img]. SB posts a blind of $0.50.
UTG calls $1, MP calls $1, Hero calls $1, <font color="#666666">1 fold</font>, BB checks.

Flop: ($4.50) 2[img]/images/graemlins/club.gif[/img], 7[img]/images/graemlins/heart.gif[/img], 6[img]/images/graemlins/heart.gif[/img] <font color="#0000FF">(4 players)</font>
BB checks, UTG checks, MP checks, <font color="#CC3333">Hero bets $4</font>, BB folds, UTG calls $4, MP folds.

Turn: ($12.50) 2[img]/images/graemlins/heart.gif[/img] <font color="#0000FF">(2 players)</font>
UTG checks, Hero checks.

River: ($12.50) 6[img]/images/graemlins/diamond.gif[/img] <font color="#0000FF">(2 players)</font>
UTG checks, <font color="#CC3333">Hero bets $X</font>