After looking at this thread, I took a look at my P4, and sure enough, I have the same thing going on with my upper legs (approx 5 hours flight time on it). On one of the legs, the 'anomaly' can clearly be seen with the naked eye, and on the other three, I had to use a flashlight to see the 'lines' by illuminating it perpendicular to the leg in order to bring out the shadowing of these 'lines'.
I then jumped on the DJI forum to see if it had any threads about this, and it did. It seems that the general sentiment is that these are mold or casting marks caused by the production process and not cracks, and I am tending to agree. I will keep an eye on this nonetheless, but I really doubt that there will be any change.