This is a differential manometer meaning that it responds to the difference in vacuum between the 4 cylinders. If you had 2 connections with leaks, the other 2 of the cylinders would read very high and try to suck the fluid in. You cannot connect only 2 for the same reason. Mercury manometers are absolute manometers, meaning that they respond individually to each cylinder's absolute vacuum, so a leak in 1 will not make another read too high.