I went to a "Bonding Without Bloodshed" workshop last night.  They stated that just bonding one at a time is best.  ie:  a pair with a single, then you can add another later.  This was just their opinion.  It might work for you to do pairs, but I think it would be difficult.

I have heard that quartets are a bit more difficult that trios. Probably because you have to deal with jealousy on ALL the bun’s behalf, and then you have to be concerned about each bunny getting along with three other bunnies. I think a trio would be much easier to do. Is your existing pair a male and female pair? The toughest part of that bond would then be bonding the two males so you might want to try introducing those two first. Advance very cautiously though because you wouldn’t want to overwhelm Butters with too much bunny attention at once!

The question about would it be unfair to bond an infected bunny with others: 

 I have had several rabbit savvy vets tell me that since it is so common in rabbits and not all get symptoms even if they are exposed,  that having a loving happy home really is priority.   And so if you can offer that, it is worth the risks.   I do agree with that on a general basis. 

I do know FOR ME, after Bailey infected Forrest (who died), I, personally don’t want to go through the risks of having rabbit after rabbit be infected.  Rucy who originally tested negative for e.cuniculi is now positive (but her titer levels are so low that it may only mean she has been exposed, but I will test her again in six months to see if the numbers have increased)  If she ends up getting it, she will be the third bunny to have contracted it.  And I don’t know about Jack.    And the thing is, I have been very careful, separate litterboxes, separate area rugs for their area, separate pens, but either it didn’t matter, or Rucy contracted it years ago when they weren’t separated.

I can’t personally watch another bunny suffer from this and so I from now on will not get another bunny until all of my bunnies who are tested e.cuniculi positive have passed, and I have thoroughly cleaned the place.

That is just my personal choice due to my own unlucky circumstances.

I know many people who have had success with a healthy bunny crew even though one was infected. 

Regarding bonding quartets.  That, I have never done, but here’s a good link to bonding multiples.

http://www.mybunnies.com/bonding.htm

Others here may have more experience with that, and if I come across some good information, I’ll be sure to let you know. Also, if you come across some good tips regarding bonding a trio to a new bunny, be sure to share it with us so we can all continue to learn.