Hi Lotte,
That should be possible with a visibility agent (hide user dimension member) with the condition:
(sum(d1, all, m1) = 0 and allcount(d1:0, d1, m1 = 1) or (sum(d1, all, m1) = 0 and allcount(d1:0, d1, m1 = 3)
Use an opposite visibility agent to hide columns if first column contains data:
(sum(d1, all, m1) <> 0 and allcount(d1:0, d1, m1 = 2) or (sum(d1, all, m1) <> 0 and allcount(d1:0, d1, m1 = 4)
Calculated columns could be modified with a Visibility modifier:
Actual Month = sum(d1:d2(v), 0, m1)
BR / Ole