OK. I switched the variables like this:
select ID, m1, m2, m3, m4,
case when m1 in ( @m1,@m2,@m3,@m4 )then 0 else 1 end +
case when m2 in ( @m1,@m2,@m3,@m4 )then 0 else 1 end +
case when m3 in ( @m1,@m2,@m3,@m4 )then 0 else 1 end +
case when m4 in ( @m1,@m2,@m3,@m4 )then 0 else 1 end...