Don't bother with the EPROM, this is a classic 'stuck key' problem.
To explain: the keys don't each have their own connection back to the controller, they are grouped together so that four or five of the keys share a common connection. The principle is called multiplexing or 'matrixing' and it makes the keyboard far easier and cheaper to manufacture. Your problem is the '4' key is stuck closed and that is preventing the signal from the other keys in the same group from being recognized.
Almost certainly as this is a 'gaming' keyboard, the membrane under the '4' key has become permanently stretched so the switch stays closed. Open it up and carefully lift out the three plastic layers of the membrane assembly, there is a top layer, a bottom layer and spacer layer in the middle with holes in it under the key positions. Look carefully and you will see a connection between all the keys that are not working. Normally, when the key is pressed, the top layer (nearest the key cap) is pushed downward through the hole so it touches the bottom layer and makes electrical contact, in your case, the connection isn't breaking when the pressure on the key is released.
Brian.