Remember, Mike Phillips is much more skilled with a polisher than most of us will ever be.
I believe that the combination of a FLEX 3401 and a 4" pad is far less than ideal.
The housing and the BP are in a precarious position when attempting to polish anything but a pillar or a spot correction.
If you plan to use this combination....at a minimum, put a few layers of electrical tape around the edge of the BP and the housing so when you do bump something, it will cause less damage.
I converted my old Porter Cable to a dedicated small pad machine. It works great with the 3" Meguiar's Microfiber Pads and the 4" Lake Country CCS foam pads. With small pads, this machine has plenty of power when used on speed 5-6.
It is also a time saver instead of having to change out the BP on the FLEX a couple of times during a job.