Maybe the question just isn't clear. Why would you want to replace 4GB of faster memory with 4 GB of slower memory? And we don't know what laptop you have, but if it's recent enough to have dual-channel memory, then you will lose half your memory speed just by going to one channel.
If the question is about putting in _two_ 4GB sticks, then you will have a long discussion thread on your hands. More but slower memory.
The most cost effective upgrade is probably gonna be 2x2GB DDR3 - 1066. 2x4GB would be better, but that will cost at least twice as much and so long as you don't have a video card and do gaming on your laptop--you won't really use more than 4GB total. What are your laptop's specs?
10600/8 = 1333MHz vs 8500/8 = 1066Mhz ; on a desktop the gain is 1~3% assuming the same CAS, bottom-line you will NOT feel any gain or loss from frequency differences. However, going from 2GB to 4GB will yield better performance and increased battery life.