Sorry to have caused some confusion....
BN2A is right : some of the pipes, pumps, generators, ancillariy gearboxes, connectors and suchlike on the outside of the 'round peg' are installed either on the left or the right side of the engine, so that they fit inside the 'square hole' while remaining accessible for inspection/maintenance.
911slf, all engines rotate in the same direction. However, the vortices rolling off the leading edges of the wings into the air intakes rotate in opposite directions.
Now if you look at a photo or a model, you'll see that the intakes of the outboard engines (#1 and #4) are quite close to the leading edges.
No problem for engine #1, because the rotation of the air entering the intake is the same as that of the engine itself, but for engine #4 there is a conflict between the senses of rotoation, leading to vibration at low speed.
Engine #3 is further back from the wing leading edge, so the airflow into the intake has already been 'straightened out' more, hence the problem doesn't occur there.
Maybe M2dude has some more details?
CJ