When a mini cargo wagon or mini fluid wagon is at the front of a train, it stops a little too early, and the alignment of the train is a little off compared to a normal wagon or locomotive in front. This means that 5 inserters can interact with each wagon instead of 4.

I did some experimenting, and the train stops in the right place if the cargo and fluid wagons' joint_distance is set to 2 (instead of 3) and the connection_distance is set to 3 to keep the same spacing. This does mean the wheels and couplers render in the wrong place, so you'd have to update the sprites to account for this.