WarGreymon is his "fourth" form. Though I consider Agumon the First form in the way I spoke of it. Maybe I misunderstood what you meant when you said that. As in Agumon is Form 0, Greymon is Form 1, MetalGreymon is Form 2, and WarGreymon is Form 3. Confusion aside, WarGreymon is pretty well known, though sometimes underutilized. It also has some variants, including Chaos and Black(they're very similar, both being dark variants. Key difference is their attribute. Chaos is still Vaccine, while Black is Virus). Others include VictoryGreymon, which is basically a humanoid Greymon with armor and a sword. Arguably ZekeGreymon is meant to be similar, as it's a golden version of MetalGreymon(2010) with fully decked out armor and no longer cyborg-like. It even combines with OmniShoutmon(to create Shoutmon DX, basically your Omnimon equivalent)., who is also dinosaur-like, but seems to be more akin to the Garurumon line in some ways, or maybe the Veemon line moreso. As mentioned above, ShineGreymon as well, being specifically RizeGreymon's Mega form. And his evil samurai form in Gaioumon(also known as SamuGreymon). There's also EmperorGreymon, who appears to be more a callback to VictoryGreymon, though is a Hybrid form for Agunimon(and yes, that name is a variation of Agumon intentionally). AncientGreymon is another Mega form among the overall series that is basically more like a bigger Greymon, but far more feral and dragon-like.