This breakout acts just like a BT keyboard, and works great with any BT-capable device: Mac, Windows, Linux, iOS, and Android. Power the module with 3-16VDC, and pair it to the computer, tablet or phone just as you would any other BT device
The MPR121 is an I2C board. You really don't need one arduino for each one. According to the datasheet, each can be set up to use i2c address 5a, 5b, 5c or 5d, so you can just hook all four of them onto the i2c bus].Alternatively, you might want you use something like this i2c multiplexer
Either way, this means that you don't need that rotary encoder. Your transmitting arduino would ask each MPR121 in turn about what pads are being touched, and it would decide what key that counts as.
Furthermore, if you want a wireless connection, the arduino might be able to emulate being a bluetooth keyboard. This would mean that you don't need a transmitting and receiving arduino. If your computer can talk to a bluetooth keyboard, well, there you go