regex - How to convert multiple repeating rows to columns and then from columns back to rows in Notepad++

I have multiple (+/- 2500) repeating rows in one column that I need to convert to a fixed set of columns which matches the repetition of the rows i.e. ABCDE-ABCDE, do my editing in column mode, then convert back to rows. This is an abridged example of my data:

A
B
C
D

E
A
B
C
D
E    

I would like it to be like this:

A   B   C   D   E
A   B   C   D   E

Then convert back to rows.