Tomb of annihilation

Ok hay fellow DMS, I was looking from some advice for this campaign, it's the Tomb of annihilation and it's something I've wanted to run but I'm not entirely sure what would be good for my group. There will be four players, I am allowing homebrew, charters and classes, but I'd like to know what you all would do.

1 Exp or milestones?

2 since its a low players amount, I'm thinking of either letting them start at level 4, or letting them bring two charters a person?

3 balancing, I want it to be a challenge but I don't want them to be instantly overwhelmed and killed.