Some additional points to consider. With the smaller diameter of the valve heads with 4 valves per cylinder the closed spring pressure can be less that with a larger diameter valve.
An advantage of twin cams is that the moving mass to open the valve is less. If the cam is directly over the valve, the cam follower cup, the valve retainer, part of the valve spring, and the valve itself have to be moved. With a single cam there is a rocker arm to move unless all the valves are in a line which brings us back to 2 valves per cylinder. In the case of a 2 valve, push rod system, there is the valve lifter, push rod, rocker arm, valve spring retainer, 1/2 valve spring, and valve to move.
With the small circumference of the smaller 4 valve heads, heat can be transferred to the valve seat more efficiently. This allows the valve head to run cooler.
Another advantage of the 4 valves per cylinder is that the valves can be arranged to place the spark plug tip in the 4 sided area between the valves. A lot of the 4 valve cylinder heads are arranged in a penthouse shape with the intake valves canted on one side and the exhaust valves canted on the other side. Not only does this allow a larger valve to be installed but also allows a short path from the manifold to the valve and allows the intake charge to drop directly into the cylinder. With the spark plug tip in the center of the combustion chamber the amount of spark timing advance is reduced to reach maximum pressure at TDC. The reduced flame travel distance also reduces the tendency to detonate allowing a higher compression ratio.
The 4 valve per cylinder arrangement allows an engine design that can safely rev to a higher RPM than a heavier 2 valve system. Individual intake valves do not have to lift as high as would a single intake valve to get the same volumetric efficiency. A 4 valve arrangement can easily buzz to 7-9 grand without valve float where a two valve push rod system requires massive closed and open spring pressures to keep the valve train in contact with the cam lobes.