So my wife will be fishing next week with a spinning rig. She's never fished for trout before. Should I load it with 4lb flouro or 6lb flouro?
Spinning rods are usually fairly stiff so they don't have the flex that fly rods have to protect finer tippet. From a tippet-equivalent perspective, you are choosing between 6X and 5X. I definitely think you should go with the heavier line.