I tried a 4x DVD-R sampler from Meritline. The Samsung and Optodisk burn at 4x and playback fine with my Sony 500A. The 4x Meritline, actually Ritek disks, burned at 2x. The disks labeled 4x Ritek burned at 4x. I have had batches of Riteks burn at 2x and batches that burn at 4x...go figure. The Princo / DVDPRO white burn at 2x, same for Accu / Lead Data. They all burnt at 4x and played back fine with my Pioneer 105. Good luck.
PS. I used to buy from ebay, but do not any more, as many of the "dealers" call their products OEM but the disks were not OEM just cheap and poor quality or poorly handled.