If your screwdriver take 1/4” bit and want to use 4mm bit. This adapter you. Or your screwdriver handle holder size is 1/4”/6.35mm and want to use 4mm bit to work. 6.35mm 1/4" Insert Bit adapter to 4mm 4 Screwdriver Bit Adapter Magnetic Holder. Whether you're tackling home renovations, electronic repairs, or automobile maintenance, this adapter expands the functionality of your existing screwdriver to meet various needs. This adapter matching with Hexagon 1/4 6.35mm handle., Manufacturer: Oxxggkao