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
aviation.stackexchange.com/users/100/Jamiec
You can add the BAe 146 to the list.
aviation.stackexchange.com/users/21881/user45726
Boeing has B747 with 4 engines.
Airbus has A380 and A340 with 4 engines.
Antonov has an-124 with 4 engines and an-225 mriya with 6 engines.
aviation.stackexchange.com/users/21882/Kennie Roy
Well... we should not forget legacy aircraft still in use.
You might also spot some Douglas DC-8 still in use.
In this link I have been able to find 8 planes still in use.
This figure via wikipedia :
In order to identify this old model just look to the engines, old fashion slim hets and look to the wing tips, there is no wing tip device.
aviation.stackexchange.com/users/6979/Trebia Project.
There's still John's 707 flying around - not commercially granted.
There's the VC-10 that could be confused with the IL-62.
Only military now but was commercial.
And the Avro 85/100.
Same but different from the BAe 146.Bit too picky?
aviation.stackexchange.com/users/6754/stuart barkley
As stated in other answers, there are quite a few more 4 engined jet liners than just the three types stated in your claim.
Identification of the 747 and A380 is pretty simple indeed and correctly asserted in the claim.
Other types will all have their own characteristics, which would have to be found by studying photos and/or drawings of the aircraft in question (and possibly knowledge of which types can be expected at a specific airport, e.g. you're not going to find Il-62s in western Europe because they're banned there for noise abatement reasons).
aviation.stackexchange.com/users/900/jwenting
Just heard following somewhere:   It is easy to distinguish four-engine passenger jetliner type, because there are only three of them:   if it has "hump" upper deck then it is Boeing 747,  if it has two decks then it is Airbus A380,  otherwise it is Airbus A340.    Is this statement true?  I'm asking about commercial passenger jetliners only. So no cargo, no military etc.
How many four-engine commercial passenger jetliner types are currently in use?
aviation.stackexchange.com
<multi-engine>
aviation.stackexchange.com/questions/38109
aviation.stackexchange.com/users/3230/trejder
There are a number of 4-engine jetliners in use in addition to the Boeing 747, Airbus 380 and Airbus 340. These include the Bae 146, which can be identified by its T-tail. Apart from the three models listed, the BAe 146 is another commercial 4-engine aircraft. Also, there are some legacy 4-engine aircraft that fit the criteria still in use, like the Douglas DC-8. Thus, this list is not exhaustive and there are more 4-engine jet liners.
Apart from the three models listed, the BAe 146 is another commercial 4-engine aircraft. There are some legacy 4-engine aircraft that fit the criteria still in use, like the Douglas DC-8. This list is not exhaustive and there are more 4-engine jet liners.