Correct. If you bet the jags with the points (4.5) and they lose by 4, then you win your bet.
If you bet $100, you'd win about $82. The standard juice on a bet is -110. Easiest way to think about this is you would need to bet $110 to win $100 or $55 to win $50 etc.
Over/Under is just total points scored in the game. If the final score was Bills 27 - Jags 23 , you would win if you bet the OVER (total points 50 being greater than 47.5) Same if the score was Bills 50 - Jags 0.