TM4C123GH6PM: using UART4 instead of UART0 for the bootloader

Hi Stefan,

  Please refer to the Bootloader user's guide for detail. To answer your question, the answer is no. ROM bootloader only support UART0.  See below excerpt. 

The supported ROM boot loader interfaces vary across devices, depending on the peripheral availability
for each specific device. UART, SSI, and I2C are the most widely supported interfaces across
devices. USB is an option for any devices that have the USB peripheral present. Ethernet is an
option for TM4C129x devices that have the Ethernet peripheral present. See the device data sheet
to determine if USB or Ethernet peripherals are available for a specific device. For UART, I2C,
and SSI, the ROM boot loader is fixed for UART0, I2C0, and SSI0, respectively. If the application
requires the use any other instance of the peripheral, then a flash boot loader must be used.
When the user application is running, it can still invoke the ROM boot loader to force an update
of the firmware through a chosen interface. The API to invoke the ROM boot loader
is ROM_UpdateXXX(), where XXX is one of the following: UART, SSI, I2C, USB, or EMAC.
For example, to download firmware using the UART peripheral, the application calls the
ROM_UpdateUART() API. These APIs remain limited to the specific ROM boot loader interfaces,
so for this example, only UART0 can be used. If a different UART channel must be used, a flash
boot loader can be used instead.