Four screws is sufficient (has been for 3 of my planes so far). They get screwed through the cowl and into the side of the firewall. Once you've got the engine mounted, you can determine how far back the cowl needs to go; dry fit it, mark the fuse with a pen where it ends, remove the cowl and measure to the center of the firewall. Then use this measurement and mark the side of the cowl from the end.