It's nothing to do with GPIO current limits. 4-20mA is an analogue signal, the Pi GPIOs are digital. You will need extra hardware (sorry I don't use 4-20mA so I have no clues), but probably connected to USB or to the i2c bus since there won't be enough GPIO outputs without a bit of fuss (multiplexing). It will be more complex than a single transistor!
@BMSDoug: that is an input board, OP wants outputs
You are absolutely right, I thought (incorrectly) that I had linked to an output board.
Two x 4-20mA output boards are available https://widgetlords.com/collections/pi- ... -interface
. This board is hardwired to use GPIO 4 as the chip select for the SPI interface, with some hardware hacking you could change the chip select to a different pin and therefor have 4 of these connected at once (getting pretty expensive).
I've not seen much else capable of outputting 4-20mA signals.
Building Management Systems Engineer.