First, welcome to FT purplecake.

CX can tag your bags through to BOS. There is a recheck counter at the exit of TBIT, so no need to lug bags over to T4. Follow the signs to "connecting flights", taking a right after customs. Just drop them with the attendant on the conveyor belt.

TBIT is pretty bad at getting bags out in a timely fashion. The only thing that makes this less of an issue is that the CBP facility is equally inefficient at processing incoming pax, so much of the time, your bags are already out by the time you finish immigration. If you have Global Entry, expect to wait a while ( / )