Not criticising; I have a 4 yr old too and she's sometimes allowed to play games on the my wife's iPad and is happy navigating the pages of apps, etc. BUT, we've been told by her nursery teacher that she's not upto speed with using a real computer with mouse and keyboard because she's too used to using a touchscreen iPad.
While it's not a major problem, most schools can't afford iPads/tablets for the pupils, so kids need to know how to use a mouse and keyboard and kids having full time access to their own tablet at home is only going to hamper their developing skills with a proper computer.
Kids also need to learn to write with a pen properly not with a finger.
Whatever you get, you need to make sure you can put parental controls on internet access and apps.
As for a tablet, the 7inch Galaxy Tab looks good, but the Kindle Fire is also excellent value and you can pick one up in Tesco.
That said, I think there are probably better apps for kids available on the iPad than for Android.