It seems to me that the fan control on a pi-top[4] is either non-existent or broken. I have tried two separate units and both behaved identically. On startup, the fan immediately kicks in at max speed and just stays there. To check this out i ran a sysbench test running all four cores at 100% for 5 minutes. The fan noise never altered once. I repeated the test using both the battery only and with the mains adapter attached, once again the fan speed never once altered.
more interestingly i ran the same test on an Intel NUC and a Mac-Mini, the sound of the pi-top[4] fan was louder than both the fully stressed NUC or the Mac Mini.
I had two long chats with pi-top support over this and the only solution offered seemed to be for me to rewire the fan and drive it via the GPIO pins.
The pi-top[4] fan control is not based upon Raspberry Pi4B thermal data, hence the pi reports 38 degrees for CPU and GPU which is well below any level of concern. They seem to be monitoring the temperature of their proprietary card using sensors that look at the ambient case temperature and the charging system temperature. This makes sense in context, the implementation is seriously flawed.
I also noticed that there charging circuit makes a high pitched whine when attached to the unit, this somewhat less than inspiring as it occurred on both units i tried. Consequently, i decided to return the replacement to Amazon for a full refund as the concept is great but execution is poor.