I'm no expert, but 4x should be perfectly fine. Nowadays, even much faster is ok for playable CDs. Maybe re-check the way the disc was finalized. Was it multi-session? That may limit where else you can play it.
Just burned my first cd and it won't play in the car, get an error message e-04, it will play in my dvd connected to my tv. I recorded it at 4x, should I go to 2x or 1x. I'm using sony cd-rw discs, my computer is an emachine t3085 with a dvd+/-rw. What's up?