If the 2004 is like mine it has a CD player and Cassette player. I've been very unhappy with the sound quality of the FM Ipod converters I've tried in the 4Runner, but I'm very happy with a Sony cassette adaptor. They cost about $20, and can be found in the Walkman section of Fry's. I first tried a Monster cassette adaptor but the rollers made noise that I could hear in the 4Runner. The Sony is a lot quieter. The first Sony cassette adaptor started to make an electronic popping sound after a year or two, so I replaced it for another $20 and have been happy ever since. I also use it to play MP3's from my Treo with a $7 stereo headphone adaptor from Radio Shack. I put the Monster cassette adaptor in the F150 and can't hear the rollers, and it sounds great.
BTW, when I was replacing the Sony adaptor due to the popping sound, I tried another brand found at Fry's for about $10 and it was absolutely, unbelievably, BAD. I tried it and returned it before leaving the parking lot, and paid the extra $10 for the Sony.