Could possibly be correct, that figure. I learnt a while ago that the world's most popular aircraft is the Boeing 737; one of which takes off about every 10 seconds around the globe. Easyjet and Ryanair use them.
Say around 1000 of those are airboune at any one time, each with a capacity of 200 people (give or take), that's 200,000 people you have in the sky already.
That's just with one aircraft type. If you take into account the rest of the Boeing family and Airbuses alone, you would probably get over half your figure.
Four million becomes more likely when you consider all the other, smaller aircraft manufacturers like Beech, British Aerospace and Fokker, flying more short haul routes etc.