What version of iOS will I get on new iPad 4 Mini I would like to jailbreak my soon to be new iPad 4 Mini. Should I assume that it would still have iOS 9 installed when I get it out of the box?
There is no way for anyone here to give you a definitive answer to that question. Although the iPad mini 4 originally shipped with iOS 9.0 (13A340), it also fully supports the latest iterations of iOS 11.

In a nutshell, you should not assume that the one you buy will have iOS 9 as the version of iOS it has pre-installed will depend on when it was manufactured. Because of this, if it is new, it's likely to have a version of either iOS 10 or iOS 11.