I had temp drops like that with the whole packer I did last week. I did open the vents on the WSM a little more to get the fire a little hotter and then the meat temp began to rise. but right now it looks like it's in the plateau. I think you are right on time. I would also put the fatties on around 4:30. Depending on the fat content, they really don't take that long to cook up. Let it sit awhile and see what the temp does. I would crank up the heat a bit if it starts dropping even more. Otherwise, it looks like you are on track.
I told you that bacon is good. You can put fresh slices on if you want more bacon at the end of the cook. You could even stuff one of the fatties with some of the chopped up bacon.