Yesterday I was listening to a live baseball game in my truck. 20 minutes later when I got home, it was extremely hot! Was using a Belkin FM transmitter/charger that I used on my old iPhone. Called Apple today and asked me to test it with an Apple charger.....I've been running streaming video from the same app (MLB) and it is staying cool. So I checked Belkin's website. They have none, ZERO of their transmitters listed as being compatible with iPhone 4. In other words, don't use it if it's not specified to be used with the iPhone 4. Looks like I'll have to get a new FM transmitter