If you have a few 4mm bits that you use a lot a drop of super glue hold a 3mm magnet on the bottom of a 4mm bit. If you want you can paint a few coats of clear finger nail polish to make it fit a 6mm CT.
A easy down and dirty way to use common 4mm bits in a CT you can buy a screw driver tip magnetizer and demagnetizer from the hardware store for a few bucks. You can magnetize the the CT bit holder in seconds and you demagnetize just as fast.
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