I'm not an expert so I may be overuled but I believe the class 10 cards are only really relevant to recording video. i.e. if you are recorded HD video say you don't want to be losing data due to not being able to save it fast enough. Playback of mp3 shouldn't be an issue for class 4.
That said why not just get class 10, doesn't cost much more, especially compared to the car. I picked up 2 class 10 64Gb cards which I use for my DSLR and transferring files around. I can put them in my TV for example to playback video saved on them. Whether I'd have any issue saving or playing back the video on class 4 I don't know but even if you can get 64Gb in class 4 the savings would be small, why have the risk?