WHERE clause is not filtering on LESS THAN and GREATER THAN or BETWEEN
The correct syntax for this would be:

SELECT DATEDIFF(M, TheDate, TheOtherDate) AS MTR 
FROM MyTable 
WHERE MyID = 2074163 AND (
    DATEDIFF(M, TheDate, TheOtherDate) &gt;= 0 AND
    DATEDIFF(M, TheDate, TheOtherDate) &lt;= 4
)