USB to UART Bridge IC question
Usually when you have a multichannel USB to UART bridge is simply appears in the system as multiple serial ports, so you don't need anything special to communicate over one of the UARTs. In Windows for example you will simply have 4 COM ports.
The datasheet says that an individual UART can go up to 12 Mbps. Each channel speed can be set separately (just like in every other serial port). USB does not have very predictible timings, but you can count on the chip (and you operating system) to reliably deliver all the data. That is what the FIFOs in the chip are for - hold the data until it can be transmitted.