The idea that we have 4 players "ready" to be promoted is probably flawed to begin with. Mayo is still striking out a ton and doesn't have a position that he can play at a Major League level. He's an awesome prospect, but he needs more time before he can come in and contribute to a winning team.
Stowers could be promoted, but beyond his early power surge he's batting .263 with a .333 OBP. I'm not entirely convinced he'll ever be an above average Major League regular. I don't think he provides anything over the options we currently have on the roster.
Norby's bat seems ready, but he's defensively limited. They've been trying to give him reps in the OF, but I doubt he's ready to play there regularly. He could hypothetically be used as a DH against LHP for now, but we might be better off letting him get more comfortable in LF / RF.
Kjerstad is the most ready to help right now, but he's blocked by Santander, O'Hearn and Cowser. We could move on from Urias, but that would limit our ability to use Mateo as a pinch runner. Would probably make more sense to bring up Norby in that case to give us more defensive flexibility. I do think it's possible that he could be better than some of our regulars by the end of the season, but it's a really tough call to move on from one of those guys on take away their ABs for a rookie. I was worried about a similar situation with Cowser, but he seized the opportunity when Hays started slow.