I think the risk would depend on the security patch level of the “Android OS” (formerly called “Fairphone OS”) that was installed on your FP4 before you switched. By the way, I guess you meant “5th of July 2025” in your second sentence, right?
Having said that, right now locking the bootloader might not (yet) make Revolut run again, as this is a known issue Murena means to fix with the very next update:
P.S.: From what I remember, changing the bootloader status will trigger a factory reset.