Having worked a two shooter wedding I can offer the following.
At least one photographer has to scout the location beforehand and find the natural backdrops and look for set ups.
One photographer has to be the lead photographer. The lead photographer is the one everyone should be looking at for the formal and sem-formal shots. The lead photographer is the one directing the other photographers to their next location. For example the lead photographer will be wanting to get the images of the bride stepping from the car at arrival. The second photographer will be getting the shots of the groom and best man anticipating the imminent arrival and will also probably get the atmosphere shots of the ushers as the car arrives. At the last one I did, I got the long distance with background arrival shots whilst the lead photographer concentrated on the step out close ups. As the lead photographer was covering the bride getting ready I covered the best man, groom and ushers before the arrival of the bridal car.
The photographers have to meet beforehand and discuss who is covering what, when and where. For example, at the ring exchange will the lead photographer be doing the close ups whilst the second does the 'down the aisle shot' and guest shots.
The second has to know when to put his/her camera down to ensure that all eyes are on the lead.
Decide who is doing the atmosphere shots. Place settings, cake, general venue, car arrival etc.
Even with two photographers who work well together and have a clear understanding of their role and the shot list it can sometimes be difficult to get a shot without the other photographer in it. With four photographers this will be very difficult (read nightmare).
I recommend two photographers max who don't have ego's they are tripping over, have worked together before and who understand that one is the lead and the other the second.
If the lead has an assistant he/she will also be texting/speaking to the second with up to date info and changes. The second will be speaking to the assistant to notify of opportunities that may not have been planned.
Get two skilled amateurs who are confident and who can demonstrate they can work together and you will be much happier with the results. Get two skilled pros and be very happy with the results (sorry, couldn't stop myself)
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One of the things myself and my associate stress is that we aim to provide "Photographs about your day. Not a day about photographs". This is important as the day can quickly turn into a series of staged and semi-staged shoots that just happens to be at a wedding.
I really would hate to have to co-ordinate three other photographers. It is hard enough with one.
As for doing it for free I am sure you will find someone but look at their work before committing yourself. There are a lot of people who really have no idea how difficult it is to shoot a wedding and will jump at the chance to try to learn at your expense. Good wedding photography is a lot harder than taking a nice picture of your mates at the seaside.
I seem to have rambled on here a bit but hopefully have contributed something as well
John