Short cables between various Arduinos (I2C)
Any jumper wire or "Dupont" connector can be used for this. 4 pin cables are easy to get, but depending on your wiring or where each pin is, ones with individual connectors may be better.

I2c is ideal for this, as you only need 50 cm max between the boards and they can be chained without needing extra pins/wire. Spi would need an extra wire for each one and a uart is really point to point, not a bus like i2c.

You may want to split power on it's own cable though. 4 amps would be too much for a single 24 awg cable. If it's 1 amp between the 5 arduinos, then you don't need to separate them.

That said, why does each arduino need to be separate? At 20 inches total distance, it seems a bit unneeded. Use a better microcontroller or combine them in one box.