I remember some of the discussion about this on the winston forum. The idea that the rod is like a 3.5 weight is because it was designed to fully load at shorter distances. Morgan designed it to be used with a true four weight double taper line. If you want to be a traditionalist about it, you'd get one of those supple Cortland peach colored double taper four weight lines.
A true four weight line, maybe a double taper with a delicate tip section, should enable the rod to do what it was designed to do. It was designed for the sweetest, most delicate presentations up close.
Have fun. You've got a classic.