Los Alamos labs looked at powering a huge cluster of RPi to cheaply simulate their supercomputer cluster . Out of that came the Bitscope blades which will mount 4 Rpi per blade and host IIRC ten blades in a single rack.
It might be worth a look. Each blade is capable of delivering up to 4A current and is compatible with DC power sources ranging from 9V to ...
The official Power Requirements of the Pi 4B is 5v, 3.0A - I.e. a total of 15W maximum power requirement from each of the Pis, so a 60W USB power supply, or larger, would be ideal. This is a maximum, as they acknowledge with:
We recommend a 2.5A (2500mA) power supply, from a reputable retailer, that will provide you with enough power to run your Raspberry ...
Buy a USB hub for charging (Anker, Belkin, etc.) and use that. No need to go all the way to the wall for multiple PIs.
This guy built an interesting stack including a USB powered network hub. https://makezine.com/projects/build-a-compact-4-node-raspberry-pi-cluster/
Some USB hubs come with a port to connect to the host device, and a power cable that plugs into an outlet to power all the USB devices separate from the host device. Many USB 2.0 hubs use this, as USB 2.0 can only supply 0.5 amps. You can probably find such a hub on amazon for USB 3.0 as well.