It is best to stay at 4X and yes... that is the recommendation speed. However, I've backed up at both 4X and 8X and they're flawless afterall. NOTE: be careful of what media you're using though, when burning at 8X. Genuine Taiyo (Fuji & Sony "made in japan"), verbatim, Maxell "made in japan" is recommend.
rule of thumb is to burn them at 1/2 the speed te disc is rated at. burn 8X at 4X burn 4X at 2X and so on. or just try setting the burning program to the lowest speed with DVD decrypter it will do the above and burn at roughly 1/2 the disc speed my 4X burned at 2.4X my 8X burned at 3.8X