It depends on how much of BA's operations are still at T4 (if any) by the time you fly. BA a/c frequently clog up the gates at T4, especially when the BA a/c using the "victor zone" (very far away) gates prevents KL from using its dedicated gate. The other delay factor is crossing the south runway if your arriving a/c lands on the north runway.

I'd say if BA (and QF) is gone from T4, your connection will be tight but comfortable as long as the incoming flight arrives on time. Transit security is within T4 and with no bussing involved (I imagine a lot of transiting pax within T4 will disappear with BA). Just remember to walk fast to the transit point (flight connection for T4, not for T1,2,3 & 5).