How to combine four formulas into one?

Sorry for my terrible description, but I have two columns with this data


A 1
A 1
B 2
C 3
C 3
D 4
D 4
D 4
D 4

How can I make it so that if say, for example, one of the D's are changed to A, it would change the corresponding 4 in the column next to it, to a 1. No new data will be added, but the values will be occasionally changed.

So now:

A 1
A 1
B 2
C 3
C 3
D 4
A 1
D 4
D 4

So basically how to make if column 1 = a then column 2 = 1, but it would have to be four formulas? I don't know if this can be done. Sorry but I hope I explained it well enough. I'm sure someone in the world has done this before and it is possible. Thank you in advance for your help everyone.

You're the one.