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Get all of the 'Endless' apps from Originator (also available for iOS). They're all free to do the first few letters/numbers/words, with in-app purchases. Some of them get a bit pricey (~$15) but the amount of entertainment and education your child will get from them is absolutely worth the cost. All of the apps focus on a specific skill development - letter recognition, basic phonics, math/counting, putting letters together in words. They are all very tactile and interactive, with really excellent and fun animations that are geared directly to a four-year-old's brain. My son could probably play any of them for hours if we'd let him. When I give him my tablet to play with he ignores all of the game apps and goes straight for these - they're that good.
The Hairy apps are also excellent. Hairy Phonics, Hairy Letters and Hairy Words. Kids ABC Train is excellent if your son is into trains as mine was.