Hi angneld,
The keyboard shortcut for Add Row(s) is option-arrow, where 'arrow' is one of the four arrow keys.
To add one row above the current row:
- Select any cell in the current row.
- Press option-up arrow.
To add four rows:
- Select a stack of four cells, ending with the one after which you want the new rows inserted:

- Press option-down arrow:

- Done.
As you've probably guessed, using the left or right arrow (with option) inserts column(s) left or right of the selected cell(s).
This was done in Numbers '09, a Mac application. I don't know if this process is supported on Numbers iCloud.
Regards,
Barry