I have been using only Arduino Uno. Now I am about to buy Mega. But a question rises. Are those 4 Serial Ports of Mega controlled still through one usb cable?
No. If Arduino ATmega microcontroller supports more than a single UART, then only the first one is available through the standard USB connection. Other UARTs use their own pins that you have to connect to on the board. Mind you these pins are TTL logic serial and need a converter for RS232 or similar. It is possible to use a second (third, ...) TTL-serial to USB cable to access the port on your PC.