Hi Onesimus,

As Toppers said either convert the info to text as you are
importing it or pre-format your cells as text before you import.

To get the numbers into blocks of four you can insert a helper
column next to your credit card numbers and insert a formula
like this into the first cell then drag it down as far as is needed.

=LEFT(A1,4)&" "&MID(A1,5,4)&" "&MID(A1,9,4)&" "&MID(A1,13,4)

If your data is not that uniform there are other approaches
along the same lines.

HTH
Martin