I'm not familiar with Parallels, but if it's like Bootcamp or WINE there shouldn't be too much hassle to run Fallout 4 in OSX. Skyrim ran pretty good with Bootcamp.

We have a thread for these types of questions. Feel free to post your question there and good luck!

http://www.gamesas.com/topic/1543446-unofficial-will-my-pc-run-fallout-4-thread-7/