Sure the estimate of what could happen if it warms 4C by 2100 is a large number.
However, we are currently looking at increases in the historical record being around 0.14-0.18 degrees C per decade. Considering there are 8.5 decades left until 2100, the math says we could expect about 1.19-1.53 degrees warming by then if the decadal increase remains constant (ie. the "pause" in increases). These numbers are not 4.
To have an increase in temperature of 4 degrees C by 2100 a positive forcing feedback must take a dominant position it the climate. We have not yet found this forcing to exist. That of course doesn't mean that it doesn't but the models are predicting it and the actuals are not (as yet) showing it.
The thing with CO2 is that increases in temperature are related to doubling of the concentration in the atmosphere. That is to say that as CO2 increases, it takes more CO2 to continue to add on more increases in temperature as concentration goes up.
In the end it all relates to forcing. Is feedback positive and large or do negative feedback loops exist that respond to increasing CO2 in the atmosphere (increased albedo of clouds for example)? There's lots of science and studies we need to do here. What we know now is not correct (the models aren't accurate) that means we need to search for more measures to include in them.