Min formula for non adjacent cells >0

Hi,

I need a formula that will test every 4th cell in a row to see if it is >0 and then returns the minimum value of the cells which are >0

eg:

A1 B1 C1 D1 E1 F1 G1 ..... Formula Result
0 20 10 30 3 5 60 =30

therefore only test A1, D1,G1 >0 then apply min to D1 and G1 (since A1 = 0 exclude)

YEEEELLP !