Yeah, concept-to-completion, four years sounds about right. But that's not what I'm talking about. They HAVE the engine. Make 120 stars worth of high quality levels. I'm talking PURE level design. It should not take that long.
Take a look at Banjo-Kazooie to Banjo-Tooie. Only two years went by between those two games, and both of them were huge, phenomenal adventures.
If/when the idea of Galaxy gets stale, THEN they should go ahead and take a few years to make a new engine/concept. But we shouldn't just drop this cool idea just because it might get old at some point in the future.