What a fascinating topic; I actually thought a while about this before I decided to reply. I have a feeling that, although people have made some excellent suggestions, it is very difficult to group all the 'fourth' members of the trio into a similarity. I can't really see Neville being put in the same category as Slytherin, likewise I can't see Slytherin compared with Wormtail.
This made me start thinking about the possibility of having an extra mind, body or spirit in a foursome, but because it doesn't balance, that extra member is ostracised. Think about Slytherin: I'd say he was an extra 'mind' - he has intelligence and ambition but little faith in anything but himself. He would therefore clash with Ravenclaw, who is the other mind in the group. However, consider the relationship that Slytherin had with the other three - after a while he didn't get on with them, his motives and goals clashed, and he ended up never feeling part of their trio. The Gryffindor/Hufflepuff/Ravenclaw trio were connected and good friends, because they made up a perfect Mind/Body/Spirit trio, whereas Slytherin upset this balance and had to go.
Think about Wormtail in relation to this. I'd say that Wormtail was a 'body', all emotion, driven by fear of others. He clashed with Sirius - the other body in the Marauder trio - but because James, Sirius and Lupin all had similar ambitions and similar confidence in themselves and their abilities (i.e. they were not as frightened and, well, stupid as Wormtail) they worked better as a trio. Thus - Wormtail had to go.
It's only a theory, but it might explain the difficulty everyone is having in classing the fourth member of the trio. Perhaps Wormtail, Slytherin and Neville actually belong to another trio that they just haven't found yet, and when they do, that will be the one in which they stay. (Well, perhaps not Slytherin seeing as he is now dead!)
Also, has anyone considered 'complimenting' in terms of relationships? Like, maybe Hermione and Ron are such a good match because Hermione's mind qualities compliment and balance Ron's body ones? Perhaps I have missed something, but has anyone thought about how they would class Ginny? I can't decide whether she's a mind or a spirit - probably the latter.