It is not clear to me from the diagram if the mark is to be cleared on port or starboard side.
From the question text I guess the mark is to be cleared on port side - am I right here?

My understanding is that mark-room should be analyzed separately for each pair of boats, at the moment the first of the two touches the zone. So in this scenario the only pair to be assessed is 1 vs. 2. For the rest you need to wait until others touch the zone. 

My opinion is that the overlap between 1 and 2 does not extend to 3 as she is not between 1 & 2. So  3 only owes mark room to 2 if they are overlapped when the first of them (2) reaches the zone. 
3  only owes mark room to 4 if they are overlapped when the first of them (3) reaches the zone. 

John

As Thomas wrote: Overlap is defined for 2 Boats. If the Mark has to be left on BB, at this time 1 reaches the zone and ist overlapped with 2 and clear ahead of 3 and 4  => 1 shall give mark-room to 2 and is entitled to mark-room against 3 and 4. If the relationship between the boats is the same, when 2 reaches the zone  2 and 3 are overlapped, 2 is the inside boat and is clear ahead of 4 =>  3 shall give mark-room to 2 and 4 shall give mark-room to 2. When 3 reaches the zone, 3 and 4 are overlapped, 4 is the inside boat => 3 shall give mark-room to 4.

As Thomas pointed out, you should look pairs of boats and apply the rules between those two boats.  This approach is true of nearly all of the rules.  This allows you to take what looks like a huge jumble of boats, break it up into all the possible pairs and then apply the rules, which makes the analysis much simpler.  In your case the pairs are: 1 v 2, 1 v 3, 1 v 4, 2 v 3, 2 v 4, & 3 v 4.  With RRS 18, then key question is whether the boats are overlapped or not when the first of the pair reaches the zone.  If they are overlapped then the outside boat has to give the inside boat room at the mark.  The room that the inside boat needs includes the room that the inside boat is obligated to give to other boats.  This is true whether or not the outside boat is obligated to give the other boat room.  The room is cumulative.

Consider the following case where Grey is added.  Since Yellow and Grey are the only pair involving Grey where one of the boats has reached the zone, it is the only pair that RRS 18 can apply to.  All of the other pairs of boats (Blue & Grey, Green & Grey, etc.) are still outside the zone so RRS 18 does not apply to them, only 11 & 12 do.  For RRS 18 we now ask ourselves whether Yellow and Grey are overlapped and they are (Blue is overlapped with both and between them) so Yellow as the outside boat must give Grey room at the mark.  Note that we have not made any determination about whether Blue is obligated to give Grey room, Blue is only relevant in determining whether Yellow and Grey are overlapped.  Even if Grey becomes clear astern of Blue before Blue reaches the zone, Yellow's obligation to give Grey room remains as that was the state of things when the first boat of the pair reached the zone.

I agree with John Ball's assessment.

John Ball is clear, correct and to the point

John C re: “Since Yellow and Grey are the only pair involving Grey where one of the boats has reached the zone, it is the only pair that RRS 18 can apply to. ”

Is there a typo in your statement above? I’m not following you there. - Ang

Ang - What I meant was that for the different pairs of boats in which Grey is involved (Grey/Yellow, Grey/Blue, Grey/Green, Grey/Purple) at the instant in time shown in the diagram that RRS 18 will be turned on for only Grey and Yellow because only Yellow has reached the zone.  As time rolls on and the boats progress forward then it will apply to other the pairs but until it does we can't say whether mark room will need to be given.  Does this help or did I just repeat myself?