I'm just going by the way they announced it in Oakland and I believe Austin too. This could have been an experiment (airlines will try out various things to see what works). It was also summer so they had a TON of families. They might use this "walking" criteria for when there are over a certain number.
I think age 4 is very generous but what would be a way to deal with a party of 10 on the same reservation with just one three year old?
I'd like to see families with car seats let on first!
so hard to walk down the airplane isle carrying a car seat and a diaper bag without bumping into the people sitting on the isle. I try to carry the car seat above the level of the seats and my diaper bag is usually a backpack, but still its tricky.
A good trick I read on the net and tried with success is to hold the car seat upside down and sideways. If it's light enough (I had a Scenera), you can even drape it over your arm. This way, you don't have to hold it up and it's at eye level with the other passengers (so they can see it coming) and as skinny as you can get it.
If you use a metal luggage cart to transport it (highly recommended and better than one of those expensive Gogo-thingies), see if you can turn it around and strap it sideways somehow. Depends on the model. Doubt this will work on a big Britax though! The Radian and the Sit-n-Stroll, for the record, can be wheeled down the aisle.