Something to note. Average core load of 4 does not mean only 4 cores are used
It could be that 16 are used but they spend most of their time on IO or idling or blocked by a lock somewhere. This is a not too rare thing to happen.
CPU load is a pretty flawed metric these days to evaluate how much you saturate the number of cores.
See CPU Utilization is Wrong and Linux Load Averages: Solving the Mystery as starting points
