I really do not know if there are any differences. I have a 2003 Pontiac Bonneville brochure and it looks the same as a 2004. The side impact air bags are not listed in the 2003 model but are available in the 2004 Bonneville. There are some minor paint color differences between the two years as well.
I have had the car for a month now and I am really enjoying the ride. What a big difference from my 95 Bonneville. I maintained my 95 Bonneville really well but this 2004 Bonneville just blows it away. I know there are a lot of members that like the older style Bonneville’* and there is nothing wrong with that. My 95 was a really nice car and I enjoyed it a lot. I hope the 2004 Bonneville treats me as well during its lifetime.
Mods so far: Wood Dash Kit-Window Tint-CATZ Fog/Driving Lights Custom Fabricated Stainless Steel Mesh Grill-Painted Calipers-3rd Brake Light Decal-Strut Tower Brace.INTENSE™ Level 1 L36/L26 Performance Package. ZZP ER Rockers and a HV3. L76 springs