MSP430FR5989

The TI Examples suite has four euscia0_uart examples here:

https://dev.ti.com/tirex/explore/node?node=AJEc6njXOCsbm..jGWdWoQ__IOGqZri__LATEST

They all appear to use P2.0/1 for UCA0. To use P4.2/3: Where you see

>P2SEL0 |= BIT0 | BIT1; // Configure UART pins
>P2SEL1 &= ~(BIT0 | BIT1);

substitute

>P4SEL0 |= BIT2 | BIT3; // Configure UART pins
>P4SEL1 &= ~(BIT2 | BIT3);

[Ref data sheet (SLAS789D) Table 6-26]