Shading Columns with Conditional Formatting

select a1 to say p500
conditional format as
=MOD(CEILING(COLUMN(A1),4)/4,2)=1 pick a colour
this will do 1-4 9-12
if you want 5-8 13-16
=MOD(CEILING(COLUMN(A1),4)/4,2)=0
only do the range you need or you may end up with "too many formats" error messages sometime in the future