BA may release award seats from 355 days out; and they indeed do on some routes.
However, some routes currently seem to be left until the 320 days mark.

Back to the OP.
Personally in your situation I'd be checking YVR as well as SEA, gives you MUCH more chance of hitting 4 CW seats on one flight. I'm finding it hard enough for 2 CW seats though.
As a 2nd flexibility tip ... you may want to think of 2xF 2xCW on the way back (if 4 x CW not available).