Castle is a way bigger gamble than most people would like to admit. I think he's a gamble worth taking, but based on historical comps, he's got a less than 1 in 5 chance of developing into a "good" shooter, and a 1 in 3 chance of being a "league average" shooter. If you're taking him at 4, it's probably unnecessarily risky to try and take raw questionable shooting at 8 like Holland, Buzelis, Salaun, Cody Williams, basically all the raw wings in this draft that I'm personally very low on. I'd try and turn #8 and #35 into McCain and Tyler Smith with a little bit of maneuvering, trading up, trading down, and packaging with second rounders.
McCain at worst is probably a bench shooter but could develop into way more than that. Play him with Castle initially to provide him with the spacing that he needs to develop his lead guard skills. He's basically diet Reed Sheppard but stronger, less good, and with less on-ball potential. His creation is probably masked currently but he can help playmake a little bit, optimistically he turns into CJ McCollum and provides off the dribble spacing and offensive output for a defensive team, and at worst in a few years once the team gets more talent he turns into a valuable bench shooter like Patty Mills, except he's a 200+ lb strong guard who can also rebound and get to the rim so he's inherently way more valuable.
Tyler Smith is a swing, but you try and get him in the late first hoping he turns into diet-Markannen or maybe middle-class Naz Reid, as he's big, can shoot, was the best player for Ignite, and despite his horrible looking tape has the best defensive metrics among all the Ignite prospects. He's also an underrated rebounder and he's young and reasonably athletic, so he's not just some Matt Bonner-type. This way even though Castle's a huge risk, at least you can mitigate some of his worst deficiencies in the high likelihood that he never develops into more than a league-average shooter.