Sounds like your #4 valve adjuster has come lose (un-adjusted its self) and therefore you will have less compression on that cylinder.
That clicking could be the lose tappet flying up and down and doing nothing.
Makes sense if you just did a valve adjust, these things are evil, they seem tight and then slacken themselves off.
Not sure about your oil problem, probably not connected at all.
I'd def lift the valve cover on #4 to see if that's all that has happened, if so all good and re-set the valve gap, hopefully the wee nuts and screws are still sitting in place.