how to read every nth row in a cell

I read a text and it produced a cell but the numbers in the table i scanned are all over the place. looks something like this.

a1

b1

c1

d1

a2

b2

c2

d2

a3

b3

c3

d3

how do i get all the a, b, c, and ds by themselves. I tried to use a for loop but I am not sure how to go about it.