Peter 4 is a Trojan virus created by Billy-G.
As a malevolent virus, Peter 4's nature is to infect and destroy any program he comes across. Able to travel through Cyberspace, he also possesses the ability to spawn multiple copies of himself.
While searching for a cure to Hitoshi's cold, Saati Namba became infected by Peter 4, who attempted to erase her core program before Hitoshi managed to destroy him.
Following being captured by Billy-G, Saati and Toeni Namba are attacked by another copy of Peter 4 but the combined abilities of Saati and Toeni easily overpowered him.
For the Peter 4 virus, Ken Akamatsu altered the name of an actual virus that was spreading at the time. He wanted to realistically convey what a virus could do, such as deleting files or hiding information.