The 9004 headlamp connectors are super weak on my 124. Melted over time with just stock bulbs. The contacts are unusually thin. Also, the wire gauge is thin compared to other cars.
If you really want to use higher wattage lights, you should at least consider a heavier wire harness, switched with relays. Some posters have mentioned that the reflectors will not hold up to the heat, however, others have reported no problems...
Actually, even with the same bulbs, just switching to a heavy harness may result in more light output, since the voltage drop I measured at the lights is fairly steep. Light output drops disproportionately fast as voltage drops. These harnesses run around $50 at various online retailers.
I have not tried this yet on the 124, but have done so on other cars with good results. Still holding out for Euros on the 124... I like running 55/100w H4 bulbs in cars with euro lamps. Does not blind other drivers on low beam. DOT just sucks all around.
Best of luck.
1986 300E 5-Speed 240k mi.