I have to use a six digit passcode because I have outlook as one of my mail accounts. I think it made me change it to a six digit when I set the outlook account up.
I only use a six digit passcode for my iPhone, but my 1Password master password is super long and complicated. I figure that’s the best of both worlds.
6 digit on my iPhone (cause I seldom have to put it in, and it's more secure) but 4 digit on my iPad mini (for no particular reason other than I haven't changed it, and I'm more likely to use the passcode on there)