The answer in a word is consistency. DB stated as a gag and Toriyama did not remove that element completely. He admitted he forgot many aspects of his story as he went on. Roshi moon busts but Piccolo was city level. Saiyan Saga Z warriors freaking out over city level explosions. Babidi read Goku as 10x planet level, base Frieza was casual planet level. Cell was supposed to be SS level, yet Buu saga Goku and Vegeta together could not stop Buu's planet buster. Not to mention everyone who stayed died from a planet buster. Not very convincing of planet+ durability. There are numerous more instances. It's simply sheer inconsistency which is why DBZ is hard to debate when going against strong characters of similar level, not that the 4 Beast is that level.