Change cell ref when formula is copied down

Hi Gurus,

Can't find a formula that would look in every row when it is copied down to let's say every fourth row. Example:

Source (Sheet1):

A1=A
A2=B
A3=C
etc.

The results would read in Sheet2:

A1=A
A5=B
A9=C
etc.

Any ideas?

Thanks in advance chaps!