How to do the sensor fusion using the 4 IMU sensors. Here, 4 IMU sensors are used to know the location of every point where the IMUs are placed? I need the location of the every point on a 3D graph where you can see the location of each 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.