Yes, Samsung Galaxy A9 does have the OTG support.
No. There is no IP rating attached to Samsung Galaxy A9.
Yes, Samsung Galaxy A9 does have the face unlock support.
Yes, Samsung Galaxy A9 does have the 3.5 mm headphone jack.
Yes, you can record 4K videos from Samsung Galaxy A9.
With that said, we'd say that the Samsung Galaxy A9 (2018)'s four cameras don't amount to a whole lot just yet — you get a little versatility and they might occasionally come in handy, but we wouldn't call this an absolute must-have for photographers. Cameras aside, this is a competent phone that delivers strong performance and looks good doing it. If you're looking for a big, bold style statement, you've found one.
The Samsung Galaxy A9 (2018) is one of those rare devices that has a clear angle going for it, in a sea of smartphone sameness. In its case, that's both a blessing and a curse. It's very easy to focus on the world-first camera count achievement and simply stop there, either writing off the phone as a one trick pony or basing your opinion of it solely on its camera capabilities.