Suggestion: Practice weakest words
I think practice should include not just vocabulary that you do not have level 4 in, but also any vocabulary you have not seen in say about 4 weeks. I know that once you fill up all of your current vocabulary you then get to practice older words, but I think they should be mixed in from the start.
I'm new to duolingo (as in: started today new), but how about a system where word levels degrade over time? That way your entire vocabulary keeps refreshing itself. Like I said: I'm new, so I don't know if such a system is already in place or not, but it sounds helpful.
The first two practice sessions after you do a new lesson seem to always be a review of that new lesson. Then it returns to practicing older stuff.
As far as I can tell you practice just the words you haven't "mastered" yet first. I have done an entire practice and only had one word for every question. Then after you've "mastered all your current vocabulary it goes back to older words. I think the 2 should be combined.