Router and internet connection?

hello,my problem is,i have 4 slots on my router 5 if you include the power slot anyhow,im not sure if i need to talk to my internet company ,ok so i have 3 computers all 3 have internet,now i want my ps 3 to have internet as well and thats the 4th slot on the router is the ps3,but the only way it gets internet,if i restart the modem and it takes away one of the computers or what ive been doing so it doesnt take away one of the computers internet,is i just unplug one of the computers and ps3 takes the one i unpluged,so like i said before is this something i should talk to my internet company about or am i doing something wrong.