If I were casting bobbers, I would overline the 9' 4wt rod with a 5 wt line to load the rod deeper. This would facilitate an easier action to propel the rig...letting the rod do the work. Rod 'action' isn't all that important as adequately loading the given rod for the task at hand. IMHO, tip-flex, fast actioned (dry fly) rods with a stiff butt are generally a poor choice for such nymphing as you propose.