At first glance I thought it might have something to do with cues, and indeed; When I deactivate “Stichnoten hervorheben” (probably something like “Highlight cues” in English), all blue and yellow boxes disappear.
But while the blue box in the Bass Clarinet is the source for the yellow cue in the Baritone, the yellow cues in the Flügelhorn don’t have their source region (Horn) highlighted. Instead, there are overlapping blue boxes in the Piccolo staff.
Something weird is going on here
One thing I can say though: I opened the project in Dorico 3.5, and it is exactly the same. So it’s not new in V4.
When the source region of a cue is on a staff that is a “singular” instrument or player (no condensing), then the source region gets correctly colored in blue.
When the source region of a cue is on a condensing staff, then the source region does not geht colored correctly, but instead the topmost staff (here: piccolo) gets colored instead.
I have a project here that shows this pattern in many variations, but it’s always the same result.
Unless I am very much mistaken, we have in fact already fixed this problem in our development builds, and the fix will find its way into the first maintenance update for Dorico 4. (This same behaviour exists in Dorico 3.5 as well, by the way.)