Size can vary depending on rider experience. A bike demo is preferable for proper fit. Correct sizing is important for safety and confidence.
Personally, once my kids turned 3 they could be in between bike sizes. I always went with the smaller size, because you can raise the seat and add risers to the handlebars. You can (almost) always make a small bike fit bigger, but you can only make a big bike fit so small. My advice is not to buy a larger bike thinking they will "grow into" it. They will have more control and ultimately be safer on a bike that's not too big for them.