M4M4BEAR = man for man for bear
Bear in this context means a hairy man who is big
While the X4X is common notation for roleplays and other sexual stuff with the forst X being the seeker, seeking for(4) the second X, it can develop into a train like here where a man is looking for an other man and a bear(man)






Semantically I would agree with you since usually the nomenclature is tonuse a + or & to denote that the seeker is looking for two or more people, however I have seen a 4 train often enough for it to be also a common way.
Also cheers for the tism