A boat may protest any boat she believes has broken a rule. If that rule is in part 2 (When Boats Meet), the protesting boat must have been involved in or have seen the incident. See rule 60.1.

Rules apply between pairs of boats. For example, if 4 boats are overlapped on the same tack, (1 being windward and 4 being leeward) and 3 does not keep clear of 4, then 3 breaks rule 11. If 2 did not keep clear of 3, she may have compelled 3 to foul 4. Likewise, 2 may have been fouled by 1. Ultimately, exoneration and penalty may pass all the way to the windward boat.

In theory, 4 could protest 3, 3 could protest 2, and 2 could protest 1. Because all protests concerned the same incident, protest committee should hear them all together. Circumstances are rarely ideal though. 4 could protest just 3, just 1, or 1, 2, and 3. If 4 protests 3, and no other protests are filed, then protest committee may (and probably should) protest 1 and 2. That way all the boats involved are parties, have the right to be present, and may be penalized.

I hope that helps,
Eric