We just bought an Isla bike for our 4 year old too.
He had tried other friends bikes and he commented straight away on getting his that it was much easier to ride, so the quality really does make a difference. It's a lot lighter, and the components are made at child's size, rather than adult's bike components put on a child's bike.
The cost is actually really reasonable long term because, in our case, we have a daughter 19 months younger it can be passed down to {most inexpensive bikes seem to have a girl or boy theme to them, so the fact this in unisex makes it more appealing too}.
Then the resale is excellent, so we will have two kids use it for an overall cost of £50 to £100 {after resale, Isla will buy them back from you, or they sell well second hand as OP has noted}.
So they're actually a bargain for such a good quality bike that is so much easier to learn on! :)