Depends on what your criteria is
In the case of my choices, the bands all had steady lineups (The Mothers had some fluctuation but were pretty stable from the 2nd album on) with members who had distinctive playing styles that significantly contributed to the nature of the music (yes, even Ringo). Whereas with solo artists, I see it being more a vision of an individual with any backing musicians varying quite a bit from album to album (or sometimes song to song).
Yeah, I'm being a pedantic dick but I assumed that John used the term "bands" because that's what he was actually looking for (and his response to this post of yours basically confirms it). And IMO it's a more interesting question than simply "top 4 music artists" but whatever.