STM32L452xx Bootloader on UART4

[SKarl.19]

I'm looking at AN2606, section 65.1 and it looks like UART 4 can NOT be used by the bootloader to load FW into the part. 

Can someone confirm that is true?

[Tesla DeLorean]

AN2606 is usually pretty definitive about such things.

Your own loader can do as it wants, and implement a standard protocol like X-Modem, that'll work out-of-the-box with many terminal apps.