The Raspberry Pi 4 supports up to 4 UART interfaces now that need to be enabled by means of an device tree overlay. You can find how to do this and which Pins are used here for now:
The RPi foundation is still preparing the documentation for this.
The preliminary Raspberry Pi 4 Model B datasheet, section "5.1.1 GPIO Pin Assignments" says:
As well as being able to be used as straightforward software controlled input and output (with programmable
pulls), GPIO pins can be switched (multiplexed) into various other modes backed by dedicated
peripheral blocks such as I2C, UART and SPI.