100NL Who raises this river 4 value?

[Speedlimits]

<font color="blue"> Villain is unknown</font>

Ultimate Bet - No Limit Hold'em Cash Game - $0.50/$1 Blinds - 5 Players - (LegoPoker Hand History Converter)

SB: $95.00
BB: $99.50
UTG: $249.05
Hero (CO): $161.10
BTN: $98.50

Preflop: Hero is dealt 4[img]/images/graemlins/spade.gif[/img] A[img]/images/graemlins/spade.gif[/img] (5 Players)
UTG calls $1.00, <font color="red">Hero raises to $4.50</font>, 3 folds, UTG calls $3.50

Flop: ($10.50) 3[img]/images/graemlins/spade.gif[/img] K[img]/images/graemlins/diamond.gif[/img] K[img]/images/graemlins/club.gif[/img] (2 Players)
UTG checks, <font color="red">Hero bets $8.00</font>, UTG calls $8.00

Turn: ($26.50) 6[img]/images/graemlins/spade.gif[/img] (2 Players)
UTG checks, Hero checks

River: ($26.50) 8[img]/images/graemlins/spade.gif[/img] (2 Players)
<font color="red">UTG bets $26.50</font>, <font color="red">Hero ?