Create a String from colums

Hello, 

I want to create a string like follows: "The opened calender weeks are CW21/CW20/CW19/CW18".

I have a column with starting with the latest calender week, like this:
CW21

CW20
CW19

CW18
CW17
CW16
CW15

...

I always need the first 4 lines (here: 21,20,19,18). 

How can i get a string just like shown above? 

Thanks!