A box mounted in the kitchen island has two 4 wire lines feeding the cook top. Can I cap off one the lines(abandon it) and connect the gas with the remaining line using the black and white and capping off the red?

The black and white should have 120 volts across when tested with a volt meter, if so, it is fine to use. However, there should a be a 2 pole circuit breaker, or two fuses, for this circuit, rated for the appliance.

It needs to be changed to a one pole breaker, or fuse, and can be rated 20 amps.