Turned set gets blocked/donked on river

[BobAvellini]

Villain is around 55/15/1.5 after just a few orbits, and he seems pretty station-ish thus far. I realize this is pretty read-dependent, but does this seem like he's blocking, or is he trying to induce a raise?

Absolute Poker No-Limit Hold'em, $0.25 BB (6 handed) Absolute Poker Converter Tool from FlopTurnRiver.com (Format: 2+2 Forums)

BB ($49.75)
UTG ($9.85)
MP ($22.35)
Hero ($57.79)
Button ($19)
SB ($12.48)

Preflop: Hero is CO with 6[img]/images/graemlins/heart.gif[/img], 6[img]/images/graemlins/spade.gif[/img].
<font color="#666666">2 folds</font>, <font color="#CC3333">Hero raises to $1</font>, <font color="#666666">2 folds</font>, BB calls $0.75.

Flop: ($2.10) 4[img]/images/graemlins/heart.gif[/img], 5[img]/images/graemlins/diamond.gif[/img], Q[img]/images/graemlins/club.gif[/img] <font color="#0000FF">(2 players)</font>
BB checks, <font color="#CC3333">Hero bets $1.5</font>, BB calls $1.50.

Turn: ($5.10) 6[img]/images/graemlins/diamond.gif[/img] <font color="#0000FF">(2 players)</font>
BB checks, <font color="#CC3333">Hero bets $3.5</font>, BB calls $3.50.

River: ($12.10) 2[img]/images/graemlins/heart.gif[/img] <font color="#0000FF">(2 players)</font>
<font color="#CC3333">BB bets $4</font>, <font color="#CC3333">Hero??