The Magic Speed is ASICS' carbon shoe for runners who are not ready to spend racer money. The 4 runs a full-length carbon plate through a midsole with FF BLAST TURBO foam in the forefoot, so you get some of the propulsive feel of a race shoe in something that will survive being trained in.
It suits runners doing tempo work and intervals who want a plate without buying a Metaspeed, and it is a sensible half-marathon shoe if you are not chasing a particular time.
At 245 g it is heavier than a real racer and the ride is firmer than the top-end shoes. It is not a daily trainer either - the plate makes easy running feel forced. Use it for the sessions, not for the miles around them.