The idea here is simply that there are probably four items in each row, so using 4n+1 will affect the first item in each row because it is targeting 1st, 5th, 9th, etc. overall.
I think the 4n part can be thought of as a modulo division matcher (matches everywhere that mod 4 = 0) and the +1 simply shifts that 1 to the right. Thus it includes all the multiples of 4, each with 1 added. And since 0 is technically a multiple of 4, the 1st item is included in this list as well.