In 2004 the Vansco multiplexing wiring system was introduced, and the ISL 400 hp engine with the two-stage Jake brake became standard over the ISC 350 engine with exhaust brake. In '03 it was still an option, and option that everybody took so it became standard! They also expanded the coach battery bank from 4 batteries to 6. Somewhere in there they switched from a 12 volt surround sound system to a 120 volt "home theater in a box" type sound system.
I bought the 37" plasma screen option when I bought mine, and I am glad I did. Lots of options for future replacement with various flat screens without having to redo the cabinets. I think the standard was a 25" tube type TV. The plasma was an option in both '03 and '04. I believe they switched to an LCD 32" screen in '05.
The '04's did suffer from the lost spoiler. Mine almost left, and a friend lost two of them. Bad fabrication of the brackets was the cause. Not sure what year that became a problem. But that is fixable. Mine has been solid since I repaired it.
Nothing wrong with '04's. I certainly would not make '03's my cutoff year.
P.S. Mine is not for sale, so that is not why I am pushing them!