I'm making a planet which hosts three distinct layers of floating islands, each higher than the last one. The islands should be reasonably thick, ranging from a couple hundred metres to three or four kilometres in average, depending on the size of the island and its relief. The layers need some separation between them of course. Taking all of this into account, it's notable the planet is going to need a lot of vertical space, furthermore, inhabitable vertical space, as the islands should house all kinds of lifeforms.
I've been playing around with Artifexian's "Earth-like atmospheres" spreadsheet of this video of his and tuned the planet so, with a similar composition to that of Earth's atmosphere, lower gravity and 4,2atm of pressure at planet's surface, we get a pressure of ~0,9atm at an altitude of 15km, which I think should do the trick for the most part.
Could complex life exist and develop in such high altitudes? Would the difference in pressure prevent life from the surface from living on the high islands and viceversa?
I'm sure there are a lot of important factors that make a difference as altitude increases I'm ignoring, like temperature decrease, solar radiation, wind currents and more. Please do point these out so I can take them into consideration.
Note that the lifeforms considered in this scenario are adapted to the conditions of their place of origin on the planet and do not strictly follow human endurance limitations when it comes to adapting to harsh conditions (i.e.: minimum and maximum oxygen partial pressure). With that said, human limitations may be used as a guide.