We bought my daughter a Leapster Explorer for her 4th birthday, and it has been well-used and enjoyed regularly since. It is a sturdy wee games console ergonomically designed so it is easy and not too heavy for small hands to hold. It has big buttons with very simple icons and labelling (my daughter worked everything out herself in seconds while her technophone mother sat reading the instruction booklet!).
It has a few simple games pre-loaded which are ok for freebies but the cartridges that you can buy separately are really great. Each game has a different educational goal so read the blurb carefully before you buy. Some focus on spelling & reading, others on sums or problem-solving. It all sounds very worthy and dull but actually the games are lovely - beautiful graphics and music - and fun too. The only thing which might annoy you is that the product is American and the voiceovers on the games have an American accent. Personally, we live with it as most of the games that dd has are Disney tie-ins and obviously the characters there are mostly American anyway.
We have found that the Explorer has really encouraged dd to see words and numbers as fun. She wasn't too keen on the idea of reading when she started school! She is now 5.5yr and shows no signs of getting bored with the Explorer. As she gets older we are buying 'older' games and fully expect to be able to hand on the console to her younger brothers in good condition when the time comes. It seems virtually indestructible.
It's great when family ask what to get her for her birthday/Christmas as we can just ask for another game (they can be pricey up to £20.
Other negatives? It does not have a mains plug with it (I believe you can buy it separately) so you have to use ordinary AA batteries. We use rechargeables and she can play for the best part of a day before they have to be recharged. Beware the case as it does not have a pocket for the cartridges to go in. They just rattle around inside next to the Explorer and fall out when you open the zip. In the end I bought a small purse for the cartridges and attached it to the inside of the case to solve this problem. The case is however big enough to store spare batteries too. I have heard that there is a new version of the case so maybe they have fixed this issue?
I was very dubious about buying any kind of games console for a 4yr old and passing if off as 'educational' but it really is and I'm very happy with it.