What is the difference between ldc.i4.s and ldc.i4?

For signed byte values, no.

ldc.i4.s is a more efficient encoding for pushing the integers from -128 to 127 onto the evaluation stack.

See MSDN

You can ldc.i4.s when your value is between -128 to 127.

ldc.i4.s is a more efficient encoding for pushing the integers from -128 to 127 onto the evaluation stack.