What bamboozle said. If I was forced to choose between a 4 and a 5, the 5 would be the choice. The 5 has less limitations than th 4. Making a delicate presentation is more a function of the leader than the line weight. Light line rods became practical to build when graphite came on the scene. When bamboo and glass were the norm, a typical weight for trout would be a 5 or a 6. The reason most of the light line weights exist now is because manufacturers want you to buy a new rod every year rather than to be happy with the ones you already own. If they offer more choices in line weights, many of us will be tempted to own them all.