The main reason is that they'd need the final manuscript by the end of August to meet a January launch, and even then they'd rather have it mid-August. Depending on my editor's reaction when she gets the first draft that could be barely achievable or completely impossible, and so April is a much more sensible choice.
(I don't particularly like January launches anyway, especially when a book has a large YA audience. Schools are closed, everyone's on holiday and all the publicity, advance copies and launch announcements have to be done in December, only to be forgotten over the long Christmas break. Hal 2 and 3 were both January-ish, so I speak from experience)
So, big apologies to those hanging out for Hal 4, but it'll be all the better for the delay. And for those of you NOT hanging out for Hal 4 - now you have more time to pick up and read the first three.
Simon Haynes is the author of the Hal Spacejock and Hal Junior series (Amazon / Smashwords / other formats)