CPU-Z shows 8 cores, but 4 of them are stopped
Is there any way of activating all cores?


You can't.

The hardware is not designed to have all 8 cores active. The reason for this is that this Octacore CPU is actually based on big.LITTLE technology, where there are two sets of four cores: a powerful big cores used for heavy processing (e.g. gaming) and energy efficient LITTLE cores used for low load applications. The system will automatically switch between the two sets depending on system load, but you can't activate all 8 cores at the same time (it'll likely overheat beyond its intended design if you somehow managed to do so).