How should I interpret the Douglas DC-4 approach climb performance chart?
I interpret the dotted line to indicate the conditions needed to meet the minimum required performance, as specified in CAR1 (the certification standards used when the DC-4 was originally built.). Conditions to the right of the line are "Good", to the left of it are "No Good".  I.E. if you had a gross weight of 60,000 lbs, and were configured as specified, the aircraft would not be able to meet the required performance at any altitude above ~6400 Ft.

I believe the "C=blah etc" statement is referencing the airspeed used to determine these performance results. C= Calibrated Airspeed, VsO² is stall speed in this specific configuration, and I'm guessing that the ".04" means 4% above said stall speed.

I hope this has only confused the question tremendously, and as solely my opinion, is worth precisely what you have paid for it. YMMV. Do not attempt at home. This offer not valid.