Always fold river against unknown?

[Spartak]

For what its worth there was a few second delay before the river bet but who knows if that means anything. Do I need a read before I call the river?

PokerStars 3/6 Hold'em (6 handed) Hand History Converter Tool from FlopTurnRiver.com (Format: 2+2 Forums)

Preflop: Hero is SB with K[img]/images/graemlins/spade.gif[/img], A[img]/images/graemlins/heart.gif[/img].
<font color="#666666">1 fold</font>, MP calls, <font color="#666666">1 fold</font>, Button calls, <font color="#CC3333">Hero raises</font>, BB calls, MP calls, Button calls.

Flop: (8 SB) 3[img]/images/graemlins/spade.gif[/img], 5[img]/images/graemlins/diamond.gif[/img], J[img]/images/graemlins/spade.gif[/img] <font color="#0000FF">(4 players)</font>
<font color="#CC3333">Hero bets</font>, <font color="#CC3333">BB raises</font>, MP folds, Button folds, Hero calls.

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

River: (8 BB) 4[img]/images/graemlins/heart.gif[/img] <font color="#0000FF">(2 players)</font>
Hero checks, <font color="#CC3333">BB bets</font>, Hero folds.

Final Pot: 9 BB