I assume that you want to show the 16 digit number 1234567890123456 to appear as 1234 5678 9012 3456 (to appear with spaces but not to have to place a space).
Format the cell/cells/rows/columns that require this by clicking:
Format, then Number, then Custom and type in 0000 0000 0000 0000.
Then those selected cells will be formatted to your wishes.
If you want 1234 567890123456 then format as 0000 000000000000