And yes - you can use the 4.0 keys with ROBOTC 3.X - you’ll just need to use the VEX Firmware Update Utility (outside of ROBOTC) to get your devices up to date.
Note: You won’t be able to download/debug over wireless with the currently VEX Cortex Master Code with VEXNet 2.0 keys. This is a hardware/firmware limitation of the keys at the moment.