Couldn't work this one out in time, I think I made the right decision but certainly not for the right reasons.

No stats on villain (just 10 hands)

Party Poker No-Limit Hold'em, $0.04 BB (9 handed) - Party-Poker Converter Tool from FlopTurnRiver.com

UTG ($5.82)
MP3 ($5.76)
MP2 ($5.22)
Hero (BB) ($5.10)
Button ($1.94)
SB ($5)
UTG+1 ($5.21)
CO ($5)
MP1 ($5.47)

Preflop: Hero is BB with J, 3
1 fold, UTG+1 calls $0.04, 1 fold, MP2 calls $0.04, MP3 calls $0.04, 3 folds, Hero checks

Flop: ($0.18) A, J, 10 (4 players)
Hero bets $0.09, 1 fold, MP2 raises $0.22, 1 fold, Hero calls $0.13

Turn: ($0.62) 8 (2 players)
Hero checks, MP2 bets $0.59, Hero calls $0.59

River: ($1.80) 8 (2 players)
Hero bets $0.85, MP2 raises to $4.37 (All-In), Hero ?
The way it played out I was thinking he had a weak ace (so we were flipping on the flop, and something like 70-30 getting 2-1 to call the turn, with some implied odds I hope).

Then I hit on the river and didn't think he'd bet into me, so I bet and he overbet pushed. If I'd called him down the whole way and then bet here, that has to look exactly like a flush.
If I'm bluffing, he only has to call, so either he thinks I've got a flush and he's bluffing, or he has Kxs (I haven't seen people play flush draws that aggressively at this level), or he has A8. Is that enough to fold?