Allright, here comes the maths lesson:
16000 rpm on a 4" cutter gives 5107 m/min cutting speed.
16000 rpm on a 4.5" cutter gives 5745 m/min cutting speed.
The m/min thing is metres per minute. This is the speed that the edge of the disc is passing through whatever it is cutting.
So as long as the rpm's of the different cutters are the same, you get a faster 'peripheral speed' with a bigger diameter cutter. The same sort of effect as when hotrod cars have huge wheels on the back.
Here endeth the lesson...
Regards,
TB
PS Al nice wood.