I was planning on using my Google Pixel 3 USB charger as a power supply and initially thought this was a big deal until I researched a bit more and realized:
1. I can just use my official Raspberry Pi 2.5 A power supply with a cheapie micro USB - USB-C adapter I use to slow-charge my Pixel.
2. This is only a concern with power supplies/chargers that use smart "e-marked" cables with active electronics in them. e-marked cables actively sense what they're connected to, and if the device reports back that it can accept > 3 A, it ramps the power up beyond the default 3 A. However it appears that non-e-marked cables have to all deliver at least 3A as part of the USB-C spec.
My charger is rated for 3 A and the cable doesn't appear to have active electronics in it. I gave it a try and it works fine powering my new RPi4. I haven't tried the official 2.5 A power supply with the adapter, but I would be surprised if that didn't work fine as well.
Perhaps my cable is e-marked as the power supply is also capable of delivering 9 V @ 2 A (i.e. 18 W) using USB-PD but it still powers my Pi without complaint.
I'm not using any power-hungry USB devices so 2.5 A will be plenty. 3 A is more than enough.
I suspect this affects people with very high power, expensive 3rd party chargers. Something like this:
https://www.amazon.ca/Delivery-Charger- ... =1-36&th=1
https://www.amazon.ca/Anker-PowerLine-D ... D9RGE0NJD6
and at those prices, you don't buy those things without researching them first.
Regarding the whole charger vs. power supply thing, I have cheapie 1 A power supply that works fine on my 1B and 2B but an otherwise reputable Pi supplier's 2 A power supply that does not power my 2B well. Funny but it feels lighter than the smaller 1 A power supply. So when I bought a 3B+ I bought the official 2.5 A power supply and it works perfectly with the 3B+.