It's interesting to sit back and watch how automotive fads from manufacturers can drift in and out of popularity. Take for example the effect that Honda's legendary VTEC system had on the performance scene. Released in the early '90s VTEC gave a kick in the pants to a lot of the Japanese small car manufacturers a kick big enough to spawn such creations as Mitsi's MIVEC, for example. Different to the VTEC, but still very similar.
Sitting on the sidelines for most of the initial period of VTEC was Toyota, who had already established a strong foothold in the Japanese small car performance scene with their legendary AE86 range and of course the venerable 4AG engine. When it comes to Japanese twin-cam four cylinders, there would be no more successful family than that of the 1600cc 4AG power -plant.
First introduced in 1984 with 87kW on tap, the 4AG impressed with its bulletproof construction, rev-happy nature and what was then strong performance. The problem that Toyota had was that while they were constantly refining their old trooper 4AG engine, all of their major competition had surpass the Toyota's power and torque numbers. The engineers at Toyota hadn't spent too long sitting on their hands though, and instead of designing a complete new engine they put the 4AG through a massive re-engineering process which lifted power outputs substantially.
Along with this engine reworking, a whole lot of high-tech 'gizmology' was added to impress the punters on the sales brochures. An unsuspecting public was shown the AE101 Corolla for the first time in 1992, compete with the new generation 4AG which quickly became known colloquially as the '20-valve'. With 160hp on tap at a heady 7400rpm, the 20-valve had the right ingredients to mix it with the B16A Honda 1600 dohc VTEC.
Searching for something a little different to the variable lift and timing camshaft controls of the Mitsubishi and Honda, Toyota chose to install an extra inlet valve. This is not quite as big an advantage (at least on paper) as some would have you believe. There were some other power adding additions to the 4AG which not much was written about, namely Toyota's VVTi variable inlet camshaft timing control and a beautiful and highly effective quad throttle-body inlet manifold.
This basics of power development remain constant over the years and Toyota adhered to them - plenty of airflow capability, high-rpm potential, and a fair whack of compression; 10.5:1! 
This is the real secret behind the performance of the 20-valve. Whether to five valve or not must have purely come down to a marketing decision for Toyota, as the concept has been abandoned in most forms of motor racing well before Toyota put pen to paper. It was a Honda engineer, 'Goto', who left his post with the Japanese giant and went to work on Ferrari's F1 program, spending a lot of time doing the numbers on four verses five valves.The conclusion reached was that the extra inlet valve did not enable the engine to develop any more power, while adversely affecting fuel economy. Goto's reasoning was highly technical and backed up by a lot of pure mathematics. Needless to say a lot of other designers listened.
There is a later model 20-valve engine known as the 'black rocket cover' version, which differs from the earlier 'silver rocket cover' model by having even higher compression (11:1) and for those lucky enough to find one with a manual attached to it, a six-speed gearbox. Some installers have had concern though with running this 168hp derivative of the engine on 96-octane premium unleaded. Some feel that the older silver rocket cover 20-valve is really just as good for street use.
There is a later model 20-valve engine known as the 'black rocket cover' version, which differs from the earlier 'silver rocket cover' model by having even higher compression (11:1) and for those lucky enough to find one with a manual attached to it, a six-speed gearbox. Some installers have had concern though with running this 168hp derivative of the engine on 96-octane premium unleaded. Some feel that the older silver rocket cover 20-valve is really just as good for street use.