This handy USB micro-B to USB-C adapter direct from Raspberry Pi Trading will allow you to use your existing with the Pi 4.  

 This small adapter allows you to convert an existing micro USB power supply (like the ) into a USB-C power supply.

Note that when a micro USB power supply is used, the maximum power that can be delivered to the Raspberry Pi 4 will be 12.5W. This means that less power will be available to devices connected to the USB 2.0 and USB 3.0 ports on the Raspberry Pi 4 than when using the new .