Who are the Four Horsemen of Death?

Back in Book 2 Whitehaven is ambushed by The Others and their walkers, led by The Others, Gerold spots 'four horsemen of the apocalypse' - 'Breaking the veil of the mist were the four horsemen of death. Each ghastly ghost leered at them as the temperature dropped beside them. They sat silent as shadows and still like apparitions. Their silhouettes were carved into the mist, phantoms at the bridge of life and death. None spoke a word as Gerold, Samalah, Silas, Edwin and Vignor bore at the four figures of darkness.'

Now from book 3 we understand that The Great Other was stood amongst them and regarded Gerold, but who are the other three. The Great Other is the main figure of The Others and the leader, being the first ever Other, but these other three are not so what are they? They must be fellow leaders of The Others, generals or something like that but their absence at Bloodraven's cave is surprising. If we understand that TGO is the leader of the entire army and these other Others are the generals then it is possible that the army of The Others is split into several different armies, led by these 3 Others. If it is so then Cephri may have to deal with multiple threats and not a centralised Other threat.

Also the existence of these four horsemen suggests that there is a hierarchy and rank within The Others, once again affirming that The Others are not just senseless followers of TGO but intelligent and complex. Now that The Winter Waste is destroyed our heroes are in trouble....