Multi-user calendar

I’m looking for a 4 tier permissions system:

1) Admin, me.
2) Authors — can write stories and add to a calendar
3) Readers — can read semi-private and public calendar entries and all stories.
4) Public, no login, can read stories and public calendar entries only.

The calendar needs 3 levels
private: only readable/enterable by levels 1 & 2 above.
semi-private: enterable by levels 1 & 2 above, readable by levels 1, 2, and 3
private: readble by all, writeable by levels 1 & 2 above.

Any ideas on models that would allow the calendar to function as planned? I’m currently using postNuke, but it’s too much extra stuff for as simple as I’d like this to be.