Hi,
We havn't experienced this as DS hasn't slept through yet, but we wereb talking about this at baby group yesterday. The HV said its common for babies to start waking in the night again between 4-5 montyhs old. It's apparently a common misconception that this means your baby is ready to be weaned, but in reality it's just normal for babies to start waking again at this time when they previously slept through. Hope that helps.
I'm still waiting for the joy of him sleeping through!
x x