If the last chick hatched within a day or so of the others, and if you had placed it under the mama during the night, it would have had its best chance of being accepted. If all this just happened, you could try one more time, at night -- but I sure wouldn't count on the chick being accepted. The best chance is to leave the eggs alone til the hen abandons the nest, then either toss remaining eggs, or incubate and hand raise in a brooder if any are viable. When you plan to hatch chicks, your best chance is to collect eggs away from the hen for a week or a little longer, then mark them and put them under the hen all at once. I believe in the ideal situation, in the wild, the hen collects eggs but doesn't sit on them til there is a clutch or group of eggs. Then she begins the sitting, which begins the incubation. This way, they will more or less hatch at the same time. The trouble, or part of it anyway, is that hatcheries have attemted to breed broodiness out of lines of chickens, to increase production.
There are no hard and fast rules about managing broodies, though. Hens are just not predictable. I have one hen who will hatch eggs then kill each chick as it hatches -- well, she did this once; I caught her killing the last chick. Of course she has not gotten to set on eggs since. And no, I don't believe it matters that a human has touched the egg or chick, not for chickens and not for wild birds, either. I think this is something parents told kids to keep them away from nests. Somewhere I read of a study someone did which disproved it, but I have no idea where, now.
