i used a 1d4 for my main camera since 2010. i got a 5d3 last year. i thought the image quality was a significant enough jump that i traded the 1d4 for an additional 5d3.
i miss how fast and sturdy the 1d4 was. it is seriously built like a tank. but overall i'm happy with the switch. having two of the exact bodies makes things very easy. 6fps has been fast enough for me to get everything i need.
this is just a personal preference, but i really, really like the silent shutter mode on the 5d. the 1d4 sounds like a machine gun at weddings. now i can have two bodies that make hardly any noise.