That code is using external interrupts to read the data. The 328-based Arduinos only have 2 external interrupts, limiting the number of devices that need to use to 2 interrupts to 1.
The Mega has 5 external interrupts, allowing you to use 2 devices that need to use two interrupts.
Or, you could get serial RFID readers and use 4 of them on a Mega or 2 of them on a 328-based Arduino, using SoftwareSerial to read one of them.