Can FSX be played online with FS2004?

yes only if you have FShostclient then you can play online games with FS2004 (you should get FShost spy to get a list of these games)
Here are the instructions.
1. Install Fshost client
2. Find a server with fshost spy
3.Open Fshostclient
4. Enter the servers ip into Fshostclient and click connect
5. Now start up FSX
6. In settings Selcect all traffic to ZERO
7. Go to the Create a flight screen and enter the game
8 Once the game loads up you should see the other players (note: they will appear as traffic)
(note: DO NOT ENTER MULTIPLAYER ON THE FSX SCREEN)