I think that's mostly because most people agree that the four elements monk is a cut below the other Monk options in terms of power. I guess it's worth comparing the two from a flavor standpoint, though, since on a surface level, they both do the same thing by adding caster elements to the Monk. Personally, though, I don't think they're very similar beyond that, and definitely not interchangeable, even for Four Elements monks that focus on the fire abilities.
The fluff's pretty definitive. Four Elements monks use their ki to harness and direct phenomena outside their bodies, and Sun Soul monks' abilities are actually the physical manifestation of their ki. It's like comparing Avatar: The Last Airbender and Dragon Ball Z (or Street Fighter. Searing Arc Strike is totally a Shoryuken).
As to Sun Soul Monks themselves, I like them. Awesome concept and cool abilities, although I'm not a big fan of their level 17 feature which I think is pretty lackluster. The rest of them make up for it, in my opinion.
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