Hi Mommy-
I am not sure about that particular brand, but I was told that for teeth development and other mouth development that straw cups, as soon as you can use them are better, I have no super expert advice other than straws are amazing....less mess, better on the enamel, and they force the mouth to pucker, sort of like when words are being pronounced. I would say go to stage 3, skip 4, because really, straws can be used at any age once they get the hang of it, and then you have regular drinking cups, so why buy stage 5? You can get small, kid sized cups at pretty much any store....wal-mart, target, the grocery, the dollar store, etc. I love the cups that are made just for the kids, but once they get to a certain age and can use pretty much anything, I am not sure it's worth the money to purchase the specific brands. Again, it completely may be worth it to buy them. But I would not worry about making sure you have each stage. Kids can go from bottles to sippee's to straw cups to regular cups, and 2 years old is perfect for straw cups.
I hope that helps
-E. M