How to compare the similarity of 2 cells?

Hi guys, I want to compare the similarity of the 4-digit primary SIC code of an acquirer, and the 4-digit primary SIC code of a target. It's for my Master's thesis.

If the first digit of each code doesn't match: similarity = 0
If the first digit of each code matches: similarity = 1
If the two first digits of each match: similarity = 2
If the three first digits of each matches: similarity = 3
If the all the digits of each code matches: similarity = 4

What is the right formula?

Example:

Acquirer SIC	Target SIC	Similarity
1896	1896	4
1963	1900	2
6952	6327	1
5369	5369	4
6321	6312	2