I wanted a Laepster 2 but they sold out at TRU. They were such a DEAL. I made the "mistake" about saying something to my mom and then SHE wanted to get the Leap Pad but they are sold out and there's no reason to buy an over priced one on Ebay.
My son is 4.5 too and has no idea what it is but I KNOW he'll love it.
From what I understand the games work from one system to the next so if and when you upgrade you don't have to buy more.
My mom got up at 2 am Cyber Monday a snagged a Leasper Explorer bundle from the Leap Frog website. She was pretty proud because it was a "Cars 2" edition. Those are sold out around here so that was a BIG DEAL to her. :)
I did mention to one of my son's speech therapists that we were getting this and she said she has nothing but GOOD THINGS to say about them. I think it's just a matter of price and how many "extras" you want to go with. The Leap Pad would have been REALLY cool but not the end of the world.