Four degrees of actual drift over several hours sounds very reasonable, unless you are very close to the equator. You can expect from 0-360 degrees drift per 24 hours depending on your latitude, just due to the rotation of the earth. I've found the error in IMUPlus mode to be much less than one degree if I start at a known direction, turn the unit around a few times, then return to the original direction within a minute or so. I agree that it is useless for inertial XYZ displacement measurements. Double integrating the acceleration, even when the unit is perfectly still results in a metre of calculated displacement after a few seconds. The orientation measurements however are extremely reliable. Bosch do call it an "absolute orientation sensor", and it does an excellent job in that role.
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