I auto-fired the river bet only to realize that I'm losing to a 5 or higher...

In fact, 33 would be the only worse hand I could have -

Poker Stars
Limit Holdem Ring game
Limit: $0.50/$1
6 players


Pre-flop: (6 players) Hero is BB with
UTG folds, UTG+1 calls, CO raises, Button calls, SB folds, Hero calls, UTG+1 calls.

Flop: (8.5SB, 4 players)
Hero checks, UTG+1 checks, CO bets, Button folds, Hero calls, UTG+1 calls.

Turn: (5.75BB, 3 players)
Hero checks, UTG+1 checks, CO bets, Hero raises, UTG+1 folds, CO calls.

River: (9.75BB, 2 players)
Hero bets, CO calls.

Results:
Final pot: 11.75BB
CO showed 4d 4s
Hero showed 3c 4h

I must say that just as funny as your bet with a 4 kicker is, it is even funnier that he called your bet with a 4 kicker!