The width is up to you.. but if the cake is 4 inches.. i don't think most ppl go over an inch wide ribbon....but again, depends on the design of the cake.. some ppl put their ribbon along the middle of the tier and not the bottom. Also if it's on buttercream,so the ribbon doesn't show the staining from the oil, some ppl saturate or rub with shortening or butter...that way the ribbon is already somewhat soaked. you can attach the ends in the back with dab of royal icing..some ppl use a large hat pin ...etc.
I usually just wrap the tier, and attach the end with a dab of buttercream. One thing though, make sure you protect the back of your ribbon so that your buttercream doesn't soak through the ribbon. I only mention this because I just had to fix a wedding cake where the back of the black ribbon wasn't protected & it was bleeding onto the white buttercream.
On a 4" tier, I personally wouldn't use a ribbon wider than about 1". I just like the ribbon to be a nice accent on the cake, and don't want it covering too much of the sides.
Hope this helps. 
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