I would say start with the aberhamic religions. make that about a year. Just basics. Fundamental beliefs. Encourage the kids to learn more on their own, if they want to. Then move on to Buddhism and Hinduism. Then on to less common religions like paganism, and wikka. Things like Egyptian, greek, and Norse beliefs already come up in mythology, and history classes.

While there is no way to teach the kids to fully understand every religion in only four years, I would say it would be a good thing to teach the basics. For instance, Not all Muslims want to blow you up. Not all Christians hate gay people. Etc.

These are common misconceptions which in our free thinking, and fast learning society, shouldn't exist.