You are so completely missing the point. It’s like the denialist claim that CO2 was ten times higher than now in the distant past, so “like, what could be da harm”? The higher CO2 levels before the mid-Pleistocene transition were part of geologically long changes in the earth’s carbon sources and sinks that played out over millions of years.
Since the MPT 1.2 million years ago, the carbon cycle has been extraordinarily well bounded between 180 and 280-300 ppm. That 100 ppm differential that you like to refer to as “a mere 4%” has been responsible for the difference between ice ages and interglacials – for the difference between mile-thick ice-sheets over North America vs. temperate climates, palm trees and swimming pools. This narrow band of CO2 excursion is an enormously powerful differential, and represents the climate regime in which the current ecosystem formed and to which it is adapted, including that pesky species known as homo sapiens. For CO2 to suddenly shoot up to 400 – with no end in sight, in a time period of a few hundred years that is geologically less than a blink of an eye – is a physical shock to both the climate system and the biological ecosystem, and the consequent destabilization and the risks that arise from it are enormous.