Oh yes it is a design fault. I could quote the engineer verbatim but suffice it to say the issue has been raised and acknowledged. The simple answer is a more vigorous fan as an extra external base fan works beautifully so there is no internal obstructions to the airflow. The fan is simply not up to the job.
then quote the engineer - at the moment you are saying little to nothing other than what you say you have heard
i accept VM are unlikely to come on here and say there is a problem but there seems to be no mass of posts about over heating hub4's
for myself i have one - its in a cool area and not enclosed in any way - i have not heard the fan run so its either totally silent or not needed where my hub is
I’ve got hub 4 and had nothing but constant faults error CS9994 which means theirs a cable unplugged but it’s not it’s due to overheating it’s now being replaced if the errors come back or that one overheats you know their mostly all bad I will update if any problems arise.