If this sounds simple it's because it is. My four year old loves to send his dad emails at work. And he's not just hitting a bunch of random keys, he's actually typing stuff.
Most for year olds' know the alphabet. Some can even write their own names. My son has a hard time writing his name. His fine motor skills are just not that great. But he can type his name. A few days a go I was writing a blog post and left to go check on one of my other kids. When I came back my son was "writing a letter to dad" He had typed his name and a some other letters. I asked him if he wanted to send his dad a real email. He said yes and I opened up the composition page on my email account.
He typed the word dad and his name and then proceeded to ask me how to spell other words and the names of some of his favorite toys. I would spell them slowly so he could locate the letters. Occasionally he needed some prompting but for the more part he was able to find the letters by himself.
This is a great activity to develop those pre-academic skill's like letter recognition your child will need for kindergarten. It also encourages communication between child and parent at a young age and may payoff when your child hits those horrible teen years.
It's important that whomever received the email writes back and you read the reply to your child. Sending a picture will make it funner.
Just a tip hit the cap lock button to start since children usually learn to recognize capital letters first.