Villain is unknown, turn I would ch/c a smaller bet but his sizing looks very strong and we can't realy expect to have value on river if we hit our boat. Anyhow is folding too weak here? He may bluff w. A 
or with something random but true my experience ppl. often make it smaller.
Poker Stars, $0.25/$0.50 No Limit Hold'em Cash, 6 Players
Poker Tools Powered By Holdem Manager - The Ultimate Poker Software Suite.
BTN: $86.49 (173 bb)
SB: $20.86 (41.7 bb)
BB: $62.13 (124.3 bb)
Hero (UTG): $50.65 (101.3 bb)
MP: $54.27 (108.5 bb)
CO: $122.18 (244.4 bb)
Preflop: Hero is UTG with 4
4
Hero raises to $1.50, 2 folds, BTN calls $1.50, 2 folds
Flop: ($3.75) 4
7 K (2 players)
Hero checks, BTN bets $2.38, Hero raises to $6, BTN calls $3.62
Turn: ($15.75) 9 (2 players)
Hero checks, BTN bets $10.02, Hero folds
Results:
Spoiler
$15.75 pot ($0.71 rake)
Final Board: 4 7 K 9
BTN mucked and won $15.04 ($7.54 net)
Hero mucked 4 4 and lost (-$7.50 net)
