Autofit columns in a range

I've done this simple code. But I want it to execute automatically?

Sub autofit()

    Range("B4:B18").Columns.autofit
    Range("C4:C18").Columns.autofit
    Range("D4:D18").Columns.autofit
    Range("E4:E18").Columns.autofit

End Sub

It's in the VBA code of a specific sheet.

Any suggestions (including improving existing code...I'm a new at this :-) )?

Regards,
Niklas