I think to an extent the 2mm is false economy as it just doesn't last nearly as well and isn't very rigid for the sides etc.
I have still got a piece of 4mm folded into a sort of tunnel outside on the back yard (I was going to throw it away but the rabbits love playing in it lol so it stayed!) that has been there for well over 18 months lol.
I found I had to use two pieces of the 2mm for a base as I wasn't happy with just one piece. It is personal preference though. But for me it will always be 4mm now.