Two ways to look at it. I include Protector in the story arc, within Known Space, that includes the specific Ringworld books you mention. In addition he's got a new one coming in the December/January time frame, Ringworlds Child (or Children). He already finished it but the publisher wants a forward to fill in plot gaps. I count the new one as number 5, but that's just how I like to view them. You wouldn't know that Protector was connected to Ringworld until near the end of Ringworld Engineers. I hope I haven't ruined anything for anyone.