What is four ounces if 16 ounces is a pound?
I suppose you're asking what fraction of a pound is four ounces. Since there are, as you say, 16 ounces to the pound, four ounces is 4/16 pound. Since 4/16 = 1/4, four ounces is a quarter of a pound. ...
What is the fraction for two ounces?
It is 2 ounces or, if you must, 2/1 ounces. If you want it as a fraction of some other quantity then you need to specify that second quantity. It should be blindingly obvious to anybody that 2 ounces, as a fraction of 1 pound, will be quite different to 2 ounces as a fraction of a ton! ...