Right size bike for my child?

I am wanting to buy my son his first bike as a surprise for his birthday. He will be four. I am struggling to decide between a 12 or 14 inch wheel. As it is a surprise, i dont really want to take him into a shop. I have measured his inside leg at 14.5 inches but i am not sure how this correlates to the wheel size though.

Age has nothing to do with the size of the bike. My guess is that he will do fine on a 16" wheeled bike as there really isn't much difference in standover height between 12", 14" and 16" wheeled models.

! would get the 14inch - he should be able to reach the peddles perfectly- I got my duaghter who was 2 at the time (leg is 11. 3/4 ) a 12' and she does perfect on it but im sure she will grow out of it this year.

Go to the bike shop. I know a few 4 yr olds riding 20" bikes. They have 16" between the 14 and 20.

Stick with industry standard bike tire sizes. 12, 16, 20, 24, 26 etc. The 14, 18 sizes and others are all new on the scene and you may find tire and tube availability is not as great. A 16" should be fine. Get a bike with a coaster brake. If it has an optional handbrake that is fine too but kids this small don't have the hand strength for handbrake to be the only brake on the bike at this age. Get a helmet and gloves and have him wearing those from the first ride so he gets used to that right away.