If your 4 yo can't or won't sit properly in a booster with a seatbelt, he needs to be in a harnessed seat still. Best practice is not to switch to a booster until a minimum of 5 years old, consistently 40 pounds (or more depending on the limits of the exact booster seat), and mature enough to always sit properly with a seatbelt. As for what your SO did, it was dangerous. Your 4 year old could have been hurt. I completely understand his thought process, but it's not worth the risk even if you're positive that scaring him would keep him from doing it in the future (which it probably won't - he's 4)
Nothing bad happened, so I would let it go. Especially since it seems to have taught him to keep his belt on 🤷 It's okay to disagree with the solution, but it's not worth an argument. Think of a solution you prefer and share that with your SO