I haven’t read all 45 pages of this thread (Sorry!), so if my contribution has already been covered, please accept my apologies.
It is my understanding that the conversion of 4AD and other prohormones to testosterone takes place only in the liver. So what is the point of applying it directly to the penis shaft rather than to other areas of the body where the skin is relatively thin (i.e., the inner thighs and inner arms and, because the skin is relatively thin and well-supplied with blood vessels, the scrotum (but watch out for alcohol-based products)). In all cases, the prohormone has to be transported by the blood stream to the liver, where it is converted to T and then redistributed to T receptors *throughout the body*. That means that no more of the T resulting from the application of a prohormone to the shaft will come back to the shaft than it would if applied to any other part of the body.
The application of Androgel directly to the shaft is another story altogether. It already contains T and therefore does not need to go to the liver for processing and is ready to go to work wherever applied; so, if T will indeed induce penis growth, it is likely that a greater concentration of it will end up in the penis.
Awhile back on a Yahoo forum that no longer exists, the owner-moderator (Ken, are you there?) reported on a theory-hypothesis-supposition that dihydrotestosterone and human growth hormone injected directly into the penis might result in growth, the theory being that the receptors responsible for penis growth might be reawakened by being flooded with the relevant hormones.
A poster on another forum dealing with PE reported on his results from a series of penis surgeries, hanging and, at the direction of his surgeon, injections of either T or hGH (I don’t remember which, and the forum seems to have shut down, at least for the time being). His gains were from about 6” in length to 8” or more and to about 6” in girth.