As far as I understand your field symbol <lfs_data> has four fields, named Header1 ... Header4, whereby in field Header4 is a deep/internal table.
You can access to it this way:
LOOP AT <lfs_data>-header4 ASSIGNING FIELD-SYMBOL(<fs>).
Or in releases before NW 7.4 SP08:
FIELD-SYMBOLS <fs> LIKE LINE OF <lfs_data>-header4. " or TYPE xyz (whatever your internal table has)
LOOP AT <lfs_data>-header4 ASSIGNING <fs>.
