Most of time, especially in summer the 6wts get a workout. Large flies like a hopper/dropper on one rod and a multiple nymph rig on the other. I have a nymph type line that turns over easy in the high winds/big water that works well. I have not tries much streamer fishing because I enjoy the extended semi natural drifts that can be accomplished fishing from the drift boat on the Yak.
4wt gets use when small dry flies come out - but that does not happen very often when I fish the water.
I'll pick up my 6wt when I through the big flies if I have it with me and rigged up. But the 5wt rods seem to do all around a good job on the Lower Canyon for me no metter what flies I am throwing. When drifting I generally keep 5 wt rods on both sides of the boat ready to go.
I even use the 6 weight up river around cle elum too. 3 weight for dries and the 6 weight for streamers are my usual weapons of choice on the Yak. I have a 5 weight but generally don't use it on the Yak, it is a very undergunned 5 (8'3" DS). I save it for smaller waters like the forks of the snoqualmie and lake fishing.
I use both a full intermediate/clear line and a short 10' or 15' (I forget the exact length of the sinking portion) type 6 sink tip. If you don't want to buy both, a type 3 sink tip is a good compromise.