Actually, I was thinking of a trio rock types and a "master" steel one that could appear in 6th gen (pure fandom lol) Its sort of like Gigalith for the shape and stuff growing on it and its expression and of course colour differentiate it. Like Water/Rock would be oval stone, with seaweed growing on it. Grass/Rock would get Moss growing on it while fire is just fire blazing on a rock. Sounds terrible, I know.
I also agree that the burst abilities are a bit overpowered, but they're restricted to those 4 so flame would go to fire/rock to deal with the x4 water weakness, water would go to the water/rock to deal with the x4 grass weakness and so on. Steel Burst is restricted to the steel one, so the distribution is really sparse.
So anyways, the quad's signature move is the rock burst, which is again, restricted to them.
Rock Burst is similar to Psyshock, where I got my inspiration from. I realised that there was only TWO special rock type moves: Ancient Power and Power Gem, with both of them hitting quite low at a 60 and 70 Base Power respectively. I thought that it would be good if there was a third skill, where I thought it'd be Rock Burst (as they're rocks! Actually the rocks are just random drawing of a Gigalith body and I decided to fit in the pan family look into it xD). I thought it'd be interesting again, if the skill has its unique mechanism so that's how I thought of Rock Burst!
So the point for Rock Burst is that:
The Fire/Rock Pokemon has a 110 base attack and 60 special attack against a Tangrowth with 125 special defense and 50 defense. Thus, when the fire/rock pokemon uses Rock Burst, it would actually be a physical move but Tangrowth will take it as a special move due to its special stats being lower and thus, due special damage (in this case) but uses attack stats!
Here are some changes I would enjoy:
Constrict - Damage increased to 30 B.P. B.P doubles every turn until it reaches 480 B.P and speed is decreased by one stage every turn when the opponent is hit by this attack
Rollout - Extra effect: Foe's Defense is decreased by one stage every turn when the opponent is hit by this attack. Accuracy increased to 100.
Ice Ball - Extra effect: Foe's Special Defense is decreased by one stage every turn when the opponent is hit by this attack. Accuracy increased to 100. Attack Type is changed to Special.