I actually bought the 4# tank from Keg Connection. Brought to local welding shop, they said it's actually a 2.5# tank, even though it has 4# painted on the side. They were going to swap me for the 2.5# tank (they do swaps only, no refills) for $11. Said they only see people swap that size every couple years. The 2.5# tanks were the same size, I don't think they were trying to screw me, but they cautioned against filling the tank to 4#.
I ignored the caution, took to local outdoors store that deals in paintball, and had it filled to 4# there for $6. Hoping this was the right move.
End story is: