I'm about to buy a dust collector that has a 4" port. I've read some
posts that suggest not using 4" ducting in the shop but to use 5" or 6"
ducting and then reduce to 4" at the dust collector and at the
machines.


What is the point of going from a 4" port on the dust collector to 6"
pipeing? Is this more efficient?

If it only has a 4" port it is probably such low capacity that there would
be little advantage to upsizing.