Dear Teddy,
Happy fourth birthday! I suppose you may be able to decipher some of these letters by now, possibly even write your own. I'm unsure when children can do that, to be honest.
It is rather strange, writing to someone in the future. In your world, Voldemort may be dead (hopefully) or Harry could be the Minister for Magic. I don't imagine he'd enjoy that though, I always envisaged him becoming an Auror or something similar. Please, if Ron and Hermione are not a couple in your future, tell them to hurry up because their relationship is tedious for us adults to witness.
I love you, Teddy
All my love, Dad
P.S. I have just realized my language is rather complicated for a four year old to understand. Ask your mother what the big words mean.