Oh I didn't realize they are only 4 weeks old, I don't personally use All Flock so I am not sure what ages it is for. I've always used Mazuri starter feed for ducklings...
Any chance you would consider just switching to a duck or all flock feed rather than supplementing niacin into chicken feed? It's just a lot easier, and much more consistent. If there is a duck feed available somewhere you need that could be an easy solution for you.