What sort of links are you bondingon the HD4? What Peplink device are you connecting to with the HD4 for bonding and how much bandwidth does that have?
Go to Status → Speedfusion and click the graphing button then run some bonding throughput tests and post a screenshot of the graph here so we can see whats going on.
You might find you need to adjust the latency cut off see more info here:
then run some bonding throughput tests and post a screenshot of the graph here so we can see whats going on.