How many four-engine commercial passenger jetliner types are currently in use?
Well, the 3 points you make are easily demonstrable to be true, and also a fairly unique feature of all three if looked at side-by-side.

The 747 is the only one with an obvious hump towards the front of the aircraft. This upper row of windows is unlike either of the other two


Photo Aldo Bidini:: source: http://www.airliners.net/photo/Alitalia/Boeing-747-243B/1200648/L?sid=f499b3169d12a0d4f410846e6512443a

The A380 has an entire upper row of windows - it does indeed have 2 full decks.


   Richard Vandervord source: http://www.airliners.net/photo/Etihad-Airways/Airbus-A380-861/2574151/L

If it has 4 engines, but neither a half upper deck and hump or a full upper deck (and it's an airbus!) then yes it is an A340


Aero Icarus source: https://www.flickr.com/photos/46423105@N03/5445375360/

However, that is not all the 4 engine passenger aircraft.

The BAe 146 is a 4 engine regional airliner, and you could add to your distinguishing features "has a T-tail, must be a 146". It is also the only one of the bunch with a high wing.


   Adrian Pingstone source: https://commons.wikimedia.org/wiki/File:Lufthansa.rj85.arp.jpg

And then we're into the Ilyushins. The Il-96, is easily distinguishable from the similar(ish) A340 as it is considerably smaller - hard to tell from a close-in photo, but in real life if one were parked next to the other the IL-96 is smaller.


Anna Zvereva source:https://www.flickr.com/photos/130961247@N06/31268448486/

The Il-86 has 4 engines but does not fit your criteria, as it only remains in service with the Russian Military, but the Il-62 is still in commercial service. It's very easy to distinguish by having its 4 engines mounted on the tail


Tim Rees source: http://www.airliners.net/photo/LOT---Polish/Ilyushin-Il-62M/1033235/L