The bigger question is if you can get into T4 after clearing customs at the TB Intl Terminal (on BA) since your flight to LAS (on AS) leaves out of T6, and if you can get into T4 then will you have enough time to get to your T6 gate?
That's not a question at all.
TBIT/ T4 / T5 / T6 are all connected post security. You can walk between them at will (allowing enough time) once you've cleared security.