IM onset is much slower and that's why you gotta give a large amount. Yeah, 4 mg/kg is considered the induction dose (IM) but is also recommended for laryngospasm. I don't have the exact source on hand but I believe onset with 4 mg/kg IM is still something like 1.5-2 min. There's probably some effect before that time limit is reached, but with that onset time in mind, I don't think you wanna mess around giving less than that to a kid who is rapidly becoming hypoxic from a laryngospasm. I would much rather give an "induction dose" and have it be more than enough than give less and have it be not effective. And a full induction dose will be gone pretty quickly anyways.