The 2XL and 3XL came with 18watt charger's. Not sure about the 4 but Google only makes 1 charger for the phones that I'm aware of. There is a 45 watt charger that's comes with the Pixel book which I have. The 18 and 45 watt chargers are available as accessories but I don't think the 45 watt charges any faster.
Just got a pixel 4 xl (will be selling my Pixel 3 xl). Wife still holding out with her pixel 2 xl until the 5 is out. Her pixel 2 xl is plugged in the kitchen counter and we have been both using it. I charged my pixel 4 xl with it yesterday and was wondering if all chargers are the same 18 watt. If so, there is no need to pull up my pixel 4 xl charger