how can you put 4ml of water in a 5ml glass. You have only a 5ml glass, a 3ml glass and water unlimited. There are no measuremnets on the glasses and the 4ml of water has to be accurate in the 5ml glass.
Fill the 3ml glass to the top - pour into the 5ml glass. Fill the 3ml glass again to the top and pour into the 5ml glass until full. This leaves 1ml in the 3ml glass.
Empty this 1ml into the 5ml glass.
Fill the 3ml glass again to the top and empty into the 5ml glass.
an alternative way would be to fill both glasses to the top and then pour out water until the water level reaches the edge of the bottom of the glasses. they are then filled to half their volume. i.e. 1.5 ml and 2.5 ml. Now add the two together to make 4ml. This method assumes the the glasses are a regular shape.