Enter data in cell and the next cell decreases value by 1

I"m sorry I'm new to this forum and I've tried almost everything possible. I researched the internet for an answer but nothing yet. If you can steer me in the right direction.

Basically I have a list of technicians that have to perform projects on a quarterly basis. Column A is labeled tech # column b is labeled visits. Now they begin with 4 visits a year. I would like to enter their technician number 636 in a2, then b2 will decrease from 4 to 3. Therefore that specific technician has 3 pm's left for the year. Also I would like to highlight the pm's while they are at 4 so when they fall to 3 the cell will no longer be highlighted. Is this possible?

Thank you,