Undo the four bolts on the circumference of the alternator casing. Probably these screws have never been taken out, it is pretty likely to ruin their head if you use a screwdriver. Do the following trick and you won't miss a single bolt as the screws are surely frozen to the alternator body.

Get hold of caliper tool and a cross-tipped screwdriver bit. Make sure that the tip perfectly fits to the screw head.