Good river fold as played. However I don't see what hands you could possibly get 3 streets of value from. When he calls the flop, I would assume he has something, since cbets on KXX flops tend to make trash hands fold. Of course he could always be a giant fish who happily calls down every street, but I would want to see some evidence for that before I act upon it.
I did a little experiment in Equilab for my personal amusement. If we assume BB would check the worst 70 % of his hands preflop (since his pfr stat is 30 %), and only call the flop with a pair or better, when the 4 hits the turn, you only have about 50% equity against his range. In this case, there is no thin value to be had. Take this with a big grain of salt, but it's just my thought process.
Board: 4♦K♥9♠ 4♣
Equity Win Tie
SB 48.96% 41.07% 7.90% { Kd8d }
BB 51.04% 43.14% 7.90% { 33-22, 4h4s, 4h4c, 4s4c, 9d7d, 9h7h, 9c7c, 9d6d, 9h6h, 9c6c, 9d5d, 9h5h, 9c5c, Ks4s, Kc4c, Qh4h, Qs4s, Qc4c, Jh4h, Js4s, Jc4c, Th4h, Ts4s, Tc4c, 9h4h, 9c4c, 8h4h, 8s4s, 8c4c, 7h4h, 7s4s, 7c4c, 6h4h, 6s4s, 6c4c, 5h4h, 5s4s, 5c4c, Ks3s, Kc3c, 9d3d, 9h3h, 9c3c, 4h3h, 4s3s, 4c3c, Ks2s, Kc2c, 9d2d, 9h2h, 9c2c, 4h2h, 4s2s, 4c2c, Ad4h, Ad4s, Ad4c, Ah4s, Ah4c, As4h, As4c, Ac4h, Ac4s, Ks8h, Ks8c, Kc8h, Kc8s, Ks7d, Ks7h, Ks7c, Kc7d, Kc7h, Kc7s, Ks6d, Ks6h, Ks6c, Kc6d, Kc6h, Kc6s, Ks5d, Ks5h, Ks5c, Kc5d, Kc5h, Kc5s, Ks4h, Ks4c, Kc4h, Kc4s, Ks3d, Ks3h, Ks3c, Kc3d, Kc3h, Kc3s, Ks2d, Ks2h, Ks2c, Kc2d, Kc2h, Kc2s, Qd4h, Qd4s, Qd4c, Qh4s, Qh4c, Qs4h, Qs4c, Qc4h, Qc4s, Jd4h, Jd4s, Jd4c, Jh4s, Jh4c, Js4h, Js4c, Jc4h, Jc4s, Td4h, Td4s, Td4c, Th4s, Th4c, Ts4h, Ts4c, Tc4h, Tc4s, 9d8h, 9d8s, 9d8c, 9h8s, 9h8c, 9c8h, 9c8s, 9d7h, 9d7s, 9d7c, 9h7d, 9h7s, 9h7c, 9c7d, 9c7h, 9c7s, 9d6h, 9d6s, 9d6c, 9h6d, 9h6s, 9h6c, 9c6d, 9c6h, 9c6s, 9d5h, 9d5s, 9d5c, 9h5d, 9h5s, 9h5c, 9c5d, 9c5h, 9c5s, 9d4h, 9d4s, 9d4c, 9h4s, 9h4c, 9c4h, 9c4s, 9d3h, 9d3s, 9d3c, 9h3d, 9h3s, 9h3c, 9c3d, 9c3h, 9c3s, 9d2h, 9d2s, 9d2c, 9h2d, 9h2s, 9h2c, 9c2d, 9c2h, 9c2s, 8h4s, 8h4c, 8s4h, 8s4c, 8c4h, 8c4s, 7d4h, 7d4s, 7d4c, 7h4s, 7h4c, 7s4h, 7s4c, 7c4h, 7c4s, 6d4h, 6d4s, 6d4c, 6h4s, 6h4c, 6s4h, 6s4c, 6c4h, 6c4s, 5d4h, 5d4s, 5d4c, 5h4s, 5h4c, 5s4h, 5s4c, 5c4h, 5c4s, 4h3d, 4h3s, 4h3c, 4s3d, 4s3h, 4s3c, 4c3d, 4c3h, 4c3s, 4h2d, 4h2s, 4h2c, 4s2d, 4s2h, 4s2c, 4c2d, 4c2h, 4c2s }
Maybe just check the turn with the intention of calling a bet, to keep some bluffs in his range. If he checks behind, he very rarely has a 4 and we can bet for value and hope to get called by 9's and PP's.