Custom Text with Formula

Im trying to do the schedule at work, and what I want is:
(attached is a picture of the cells)

To be able to enter '4:00 PM' into cell B4, and have it show up as 'CL', but do the formula as if I had entered '4:00 PM', giving me the difference in hours in cell C4.

But if I enter any other number than '4:00 PM', I do not want 'CL' to show up.

How can I make this work?
Thanks