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.