I just completed the 4th puzzle, the optional objective was pretty good and I felt clever when I figured out what to do.

However, I encountered an issue I'm not sure you expected. I got to the point where both the electric mining drill and rocket launcher were being manufactured. But the mining drill finished first and the rocket launcher stopped building because I 'completed' the objective. I ended up having to play around with swapping out regular and fast components until I hit a combination where the rocker launcher finished first.

Is this expected behavior?

Yes, it is expected behavior. I use this feature on several other levels. Typically player has to speed up the optional objective or slow down the primary one. For example, he has to build a long twisting belt to slow down resources for the primary objective or use normal inserter instead of the faster inserter. I know this behavior should be not very intuitive. I wrote it to rules (button "Game Rules" bullets 7 and 8), but I try to add more clarification in the next release.

The optional objective of level 04 is very hard. The player has to find at least six tricks to solve it. Maybe you're the first one who did it :)