Boston's fourth-round selection is right-handed pitcher Ty Buttrey. The high school product is already 19 years old, and has hit the mid-90s with his fastball. He was a starter, but also relieved in between his starts, which Baseball America contributes to his velocity drop as the spring progressed.
He was selected at #151, but Baseball America rated him #38 in their top 200 -- concerns about signing him exist in the sense he has the leverage of college baseball on his side, but this is where Boston going after an inexpensive, high-floor player at #31 helps, as it should give them some wiggle room to go over slot for a fourth-round selection like Buttrey.
Why not just draft him at #31? Because if Buttrey failed to sign, the Red Sox would lose a sizable chunk of their draft budget, since those are based not just on record, but also on number (and placement) of picks in the draft. If Buttrey fails to be swayed as a fourth-round pick, it's less of a problem for Boston's MLB-imposed budget. Via Brian MacPherson, failing to sign at #31 would cost the Sox $1.575M of their $6.88M budget, whereas at #151, they would lose under $300,000 from the pool.
He works fast -- something we're not quite used to from Boston pitchers these days -- and in addition to his fastball, also throws a knuckle-curve that sits in the high-70s, as well as a change-up. Like everyone besides Brian Johnson selected to this point, it's not clear whether he's going to be a starter or reliever in the long-run, but at #151 he was clearly one of the top remaining arms in the draft.
His delivery isn't consistent yet -- not a huge surprise for a teenager -- and while his change doesn't get high marks, MLB.com thinks that, with more use, it'll become a consistent offering for him, giving him that third pitch he needs to start.