At least, that's what the editors of TurboPlay magazine suggested all the way back in 1992--just before Taito's Mizubaku Diabouken (aka Liquid Kids) hit the streets in Japan.
My initial reaction to that suggestion was something along the lines of "nuh uh!"--but after giving it some consideration my reaction has softened a bit.
After all, the series' other (actual) entries--Bubble Bobble, Rainbow Islands and Parasol Stars--don't share enemies, protagonists, settings or weapons, so why would part four--with its waterbomb-wielding platypus--be any different?
All that said, Mizubaku Daibouken isn't, as far as I can tell, officially called chapter four of the Bubble Bobble saga--although I suppose that may have been something the game's creators considered early on.