Uniqueness based within another column choice

Hi, have a unique problem,

Column A

Number column which needs a unique 4 digit number. 

Column B

Choice column with multiple choices. X, Y, Z

There are 4 X, 3 Y and 2 Z. items in the library

all the X items need column A values to be unique to other X items

all the Y items need to be unique to each other

I guess i need a simple validation to check column A uniqueness with other similar column B values. and return invalid if not the case. 

Am I correct in thinking this is possible, I've not tried validation before and not aware of the code required.