Old truck and iPhone 4?

Is it the radio that's broken or the stereo? If it's the radio, and the chevy has a cassette player. You can get a cassette adapter. You put it in your stereo's cassette and run the aux from the adapter into your iphones 3.5mm jack. You can get one for around 5-20 dollars. However, it doesn't charge your phone.

You can get an FM Transmitter, but they sound awful. I don't think there is a way around it if your stereo is broken.

You could always get a new head unit(stereo) That is honestly your best bet in the long run. You will get tired of bad quality from transmitters or adapters. The cheapest ones I've seen so far are the ones at auto zone, and advanced auto for 50 dollars. Walmart has some deals also. You will just need to purchase the stereo, and the dash kit with the wiring harness. It's all in one box. For around 75 dollars you've made the best investment in sound quality that you can.

Good luck!