Depending on the size of the four year old, I'd recommend going straight to the Maxi Micro. My son is pretty small for his age and was still too tall for the Mini Micro by the time he was five. As soon as they get too tall, their centre of gravity is too high for the scooter and they start falling off a lot. I had hoped to get double use out of the Mini Micro because he has a younger sister but she is quite happy on a Maxi Micro aged three. Hope this helps!

What age range is a mini scooter good for? If you have a four year old, would it make sense to get the next level up?

Mini Micro Scooter - Blue.
Very good condition. One reasonably careful owner!
Suitable for children up to around age 5 (max. 20kg). It has been a lifesaver on the school run but we have now traded up to a Maxi Micro.
£25
Pick up from SW15 - Putney.