I plan to run three or four Arduino Nano at the same time. Can we use a single power adapter 5v 2.0A or PC power supply 5v 22.0A on all four Nano at the same time? It is my understanding we need to disconnect the power before use USB to connect for download software.
microguy:
I plan to run three or four Arduino Nano at the same time. Can we use a single power adapter 5v 2.0A or PC power supply 5v 22.0A on all four Nano at the same time?
Naturally. Try and keep the power wires running together as pairs and as short as possible.
microguy:
It is my understanding we need to disconnect the power before use USB to connect for download software.
It is recommended as if your 5 V power supply is running significantly high or the PC is turned off, it may damage the PC and particularly laptops by feeding power back into the USB interface.
That said, most powered USB hubs do connect the 5 V supply directly to all USB ports including the input and rarely cause problems. 
Hi,
I design projects that usually have external power supplies for the Arduino controller.
I have a USB extension lead that I cut in the middle and in a small case reconnected the gnd, D+ and D- wires, but left the 5V wires disconnected.
This way with gnds connected, you can communicate with the controller while it runs on another supply.
The other solution is to purchase an "Isolating USB" lead or module or HUB.
Tom.... 
I do it with a port expander. I power all of the nano's with one external power supply. The tricky part is keeping straight what nano is connected to what port. I do not disconnect power when downloading software, I also use the serial monitor. I use Linux Mint on a desktop computer; I currently I have 8 workspaces so I keep each Nano in its own work space.