I think you're pulling that figure of "4 million" out of your arse. No back up. No documentation. Just speculation. It's much more reasonable to accept their stated membership of 8 million. They do appeal to the downtrodden and they are making babies just as fast as everyone else these days.
More important in my mind is how many of the 8 million are "fictitious"? By that I mean, HOW MANY ARE NOT REALLY PUBLISHERS!
When the CO comes through, the Secretary calls Joe Irregular and gets Joe's field service time "up to date". Joe says he probably had 4-6 hours every month but just forgot to report it. SIX reports go in as six different publishers in that Cong record and not Joe is a regular publisher, in good standing. Bullshit. Joe maybe shows up one Saturday per month, says he has "his own group", and they go to McDonalds for a couple hours and have breakfast after making one "Not At Home" call and making one more as they leave McDonalds.
It includes all the PIMO's who turn in a token Field Circus report. It includes all the kids that just "ride along" all day, as well as all the adults who just ride around all day.
So in reality, there might only be 4 million WITNESSES who are really attempting to accomplish anything. Or less.