I don't recommend a computer at age 4. My 4-year old jumped on our laptop screen (SXGA, a $700 repair). I mail-ordered a new screen from Malaysia and repaired it myself (cost: $240).
At age four, I highly recommend www.headsprout.com. My kids went through it and then began to REALLY AND TRULY READ. We gave them NO COMPUTER = NO TV = NO VIDEO GAMES, and by 2nd grade, they were reading 30,000 pages per year. That's what NO COMPUTER will do to your kid ==> Make them more engaged in reading and that will make them smarter !!!