I think that it should really be 12mths plus, because looking at the dimensions they are pretty much the same as the scuttlebug (floor to seat height / weight limit), I suspect like many companies now, they put 3yrs in rather than 12/18mths to put the onus on parents if their child falls off if they're younger than 3.
At 4yrs, think a balance bike may be better, having said that my youngest who's 6 in 2 weeks, still re-discovers her scuttlebug every so often & plays with it non stop for a few days. I keep meaning to donate to playgroup, but then see her enjoying it again & relent (and she has a bike, scooter, go kart etc, so it's through choice she uses it)