Sensor fusion using IMU sensor
Sensor fusion means using the input of multiple (different or similar) sensors to reduce the error compared to using only a single sensor. For MPU-9250, sensor fusion is typically done combining gyroscope, accelerometer and magnetometer data to get to an accurate representation of the orientation of an object in space, rather than its position. 

Perhaps - depending on how the 4 IMU are physically related to eachother in space - you could get an approximate idea of their relative positions using different sensor fusion algorithms, but as far as I know, IMU's are typically not used for that.