What you are asking about are vacuum hoses, not vents. Each carburetor has one in the same spot. As you note, two of them are connected via a T fitting to the valve in the emissions system. That system introduces clean air from the air filter box into the exhaust ports through some reed valves on top of the engine in order to reduce exhaust emissions.
The port on one of the other carburetors is connected via a vacuum hose to the fuel valve (petcock) on the bottom of the fuel tank. That valve automatically cuts off the flow of fuel to the carburetors when the engine is not running.
The fourth vacuum port is sealed off with a rubber cap.
The four ports are also used to synchronize the carburetors as part of a tune-up.
You need to get a genuine Kawasaki factory service manual for your exact year and model. You can download a PDF version for free. It is located under the Articles tab here at KZR.
You also need to get a proper set of screwdrivers. All those screws that look like Phillps head screws are actually JIS cross heads. A Phillps screwdriver can really mangle the screw heads when the screws are really tight. You can modify some Phillips drivers by carefully grinding the tips, or you can buy a set from vendors like Z1 Enterprises. There is a link to them on this page.
