Look at TRM, dedicated pin limitations.
These are the dedicated pions, meaning e.g. the fixed-function SPI component can use only certain pins. But UART is not in here. The hint you are looking for is in the architecture TRM, part 7.17:
Port 4 does not have a dedicated port-adapter (Port Adapter Block on page 161). Therefore, none of the port 4 pins can be routed through the DSI.
UART is part of the SCB, so makes one wonder if table incomplete. You raised indirectly
an interesting question. I will file a CASE on this.
I ran into similar problems - but the UART-SCB(!) can be connected to P4.0/P4.1.
However, on PSoC you've to be patient with the pin settings for other components. In my case I ran into trouble because of different input level thresholds. So, if you upload your project I can take a look into it.