3 of those 4 guys had already had at least a month of service time, so I imagine they didn't want t wait until June. Also, it appears the Sox have a different opinion on the acclimation of SP's than they do position players, bringing up Kopech and Cease, while leaving Eloy and Robert down.
For sure, Collins has got to be coming up after rosters expand. He'll be 25 next year, time to see what you've got.