Assuming that you want the result in the same field where you entered the number, you can set up this auto-enter calculation:
Let ( n = Filter ( Self ; 9876543210 ) ;
Left ( n ; 4 ) & " " & Middle ( n ; 5 ; 4 ) & " " & Middle ( n ; 9 ; 4 ) & " " & Right ( n ; 4 )
Be sure to clear the "do not replace existing value.." check box and keep in mind that this will not update existing records automatically, only records where you edit the value in this field. Updating values in auto-enter calc fields without using Replace Field Contents
That's great, thanks Phil.
I wanted it in a Calculation field, because I was substituting some other characters as well, but I got that sorted.
Thanks very much.