4= number of holes
108= distance between the holes in mm i.e. 108mm apart from centre of hole to centre of hole opposite on 4 stud
63.4== the centre bore of the wheel on the mounting face.
ET is the offset which is the distance from the vertical centre lineof the wheel to the surface of the wheel mounting face measured in mm
The smaller the offset the further apart the wheels ie et15 will stick out away from car and et 35 will be 20mm inwards- ideally where a sierra should be.
You can obviously fit the et 15 on but it will widen the track and may upset the handling and where tyres prematurely
Also the important is to measure the centre bore . If it is bigger than 63.4mm then the wheel will 'float' and not locate properly. If it is to small the wheel will not sit flush to the hub