I have a 2018 9.7" iPad, an iPad mini, and 2 iPhone X (soon to be XS or whatever it will be called).
Is my PowerCore enough? If not, what should I get?
Can 20100mAh charge all 4 devices before needing to recharge the PowerCore itself?
20100mAh should be enough to charge both of your iPads, but it will likely be out of juice before it can charge your iPhones. So you may need to grab a second PowerCore if your intention is to charge all 4 devices without charging the power banks.
As @TechnicallyWell stated you may not be able charge everything up before the battery pack needs a charge. iPads needs a lot of power to charge.
I have an old ipad 2nd gen I use for photographer related stuff only and I can get 2 charges off it maybe 3 (I can't remember) before I have to recharge.