1 Port keeps clear of starboard

You are on starboard when the wind is on your right side.  Equivalently, your main boom lies over the left side of the boat.  This is relatively straightforward when beating into the wind.  

On the run, however, the wind might not be clearly on one side or other of a boat, and in this case the main boom tells you which tack you are on.

Regardless of the rules about "windward / leeward" and "ahead / astern", the starboard tack boat has right of way over the port tack boat.

2 Windward keeps clear of leeward

You are leeward when the other boat is closer to the direction the wind is coming from than you.  Equivalently, the windward boat's main boom is pointing towards you (and you are leeward if your main boom is pointing away from the other boat).  Again, this is relatively straightforward when beating.  

Sometimes it may not be clear which boat is closer to the wind's direction than another, particularly when the boats are both running.  In this case, the leeward boat is the boat whose main boom lies away from the other boat.  In the illustration on the right, although the red boat "seems" to be to windward, it is in fact the leeward boat.

This rule applies when the two boats are on the same tack, either on port or on starboard.  (If the boats are not on the same tack, then the "port / starboard" rule applies.)

3 Astern keeps clear of a boat ahead

You are ahead when the other boat is completely behind a line drawn at right angles to your transom.

This rule applies when the two boats are on the same tack, either on port or on starboard.  (If the boats are not on the same tack, then the "port / starboard" rule applies.)

4 Give room to (all) the inside boat(s) to round the mark

This rule applies starting at four boat-lengths from the mark.  This area is called the "zone".

A boat is inside if it is not completely behind a line drawn at right angles to your transom.  The question of whether it is inside is judged when you are four boat-lengths from the mark.

There are a number of wrinkles to this rule when you read it in detail, but this is the gist of it.  The inside boat is entitled to mark room all the way round the mark while you remain in the zone. 

This rule over-rides the "windward / leeward" rule.  The inside boat always has mark room.

The boat which is ahead when it enters the zone is entitled to mark room, even if another boat gains an overlap inside the zone.

Now the "port / starboard" rule counts as long as the boats are beating to windward on opposite tacks even when one or both enters the zone.  Put simply, there isn't a zone when boats are on opposite tacks approaching a windward mark.

As John Ball notes, however: "There is a zone for boats on opposite tack (gybe) at a downwind mark", and so this rule over-rides the "port / starboard" rule when running boats on opposite tack arrive at the zone of a leeward mark.