If you're putting bees that have never been on 4.9mm foundation before, you're due for a mess. If this is the case, I'd use starter strips first, and then move to 4.9mm in the next generation. What you're going to need to do is what is refered to as "regressing". That is, get bees who were born in large cells to draw small cells. They seem to only be able to draw 0.2-3 mm smaller easily. If forced onto foundation outside of that limit, they'll probably mess it up. So the first round of foundation is sacrificed in the process. You'll cut it out and replace it.