There is at least one known major difference between FS2002 and FS2004 and that is that their GPS systems are incompatible. The Tecnam Sierra Amphibian was designed for FS2002 but, if you know what you are doing, you can make it fly under FS2004.
The Sierra Amphibian uses the native GPS for FS2002 in conjunction with the "gps500x.gau" gauge. The trick is to replace the "gps500s.gau" gauge in the panel.cfg file of the Sierra. The GPS portion of the Sierra panel.cfg file looks like this (for the 2D window):
The virtual cockpit does not actually have the gps500x.gau.
What you see in the flight simulator is just a "dummy"
picture. For some reason this gauge does not work in the
virtual cockpit so I tricked you.
If you want to replace the "gps500x.gau" gauge with the native FS2004 GPS gauge, go to the panel.cfg file for any aircraft that you currently know flies with the native FS2004 GPS gauge. Open this panel.cfg file with Notepad. Write down the name of this GPS gauge exactly as it appears for the FS2004 native GPS. There will be numbers after the gauge name and these are location and size numbers. You might want to write down these location and size numbers also. If you use these numbers your GPS will have the same location and size as the plane from which you took this information. You can alter these numbers if you wish for your own location and size requirements. Trial and error must be done to do this.
Next go to the panel.cfg file for the Sierra and open it with Notepad. Find "gps500x!gps" in the file and erase only the "gps500x!gps" part and then replace it with the exact name used for the FS2004 native GPS. If you wish you may change the size and location numbers to those used for the native GPS. Do not forget there are two places to change the "gps500x.gau" gauges for this panel.cfg file. Now be sure to hit "Save" in order to make the changes permanent for the Sierra panel.cfg file.
You might be able to find a Garmin 500 (or the equivalent) that is compatible with FS2004 on the internet. If so, you might want to use this instead of the native GPS from FS2004. Download the GPS gauge from the internet and install it the way they tell you to do it.