Hi Paul, Welcome to posting on the Forum!
There is no standardized process to determine fly rod line weights. Every rod rating is simply someone's opinion. Your individual casting style makes the biggest difference in which line works best with any given rod. What might be a 5 weight rod to me, could be a comfortable 4 weight to you. You won't lose your fishing license for using something other than the marked line weight.
TL Johnson rods are relatively stiff compared to more full flexing rods, such as a McFarland. Sounds like some experimenting is needed. As a long ago lab supervisor once told me, make
big changes, not little ones. If a DT4 line overloads the McFarland, then try 2 and/or 3 weight lines. For the distance you might comfortably cast a 6'3" rod (see
this post), there is little difference between a DT4 and WF4 line (especially one of those overfed Rios).
Hook & Hackle is not selling their branded fly lines anymore.
Tom