4AD just was a diol derivative of testosterone (two alcohols, one at the 17b position and one at the 3 position whereas test has a n alcohol at the 17b position and a ketone at the 3 position).
So the alcohol at the 3 position was reduced to a ketone so, it went from C-OH to C=O and became testosterone. 4AD was pretty active in itself, and that's why it worked so well in transdermal solutions, not because it was oxidized into testosterone. 4AD is to Test as 1AD is to 1-Test in this regard.
This talk about "pro-steroid, pro-hormone" etc is all kind of silly, honestly. Pretty much anything that is an androstane derivative is technically a steroid, including DHEA and other PH's, etc.
Here is androstane, and it is numbered so that you can see the positions (1,2,3,4,etc) that the nomenclature on the back of your bottle is referring to.
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Now go look up testosterone and the structures of other steroids, and even DHEA, estrogen, cortisone, etc. You'll see why they're all considered "steroids".
Notice where it says 17. alpha (a) and beta (b) are just stereochemical descriptions positions of the substituents, so don't worry "too much" about that when I mention it. The 17b alcohol is important (I don't know why, but its very important for AAS). If you don't "protect it" it will be reduced (proton "H" removed) to a ketone. Adding the 17 alpha alkyl group (in our case, this alkyl group is a methyl -CH3) pretty much eliminates the opportunity for the -OH to be reduced to a ketone because its hard as hell to make alkanes (like -CH3, aka methyl) "leave".
1AD itself may not have been that potent, but because its 3 position alcohol could be easily reduced to a known compound (1-Test where the 3 position has a ketone) that's probably why they called it a prohormone, because its target hormone was the one that's doing the leg-work.
If 3AD is active on its own or not isn't really a factor in if its considered a steroid. By default, its a steroid. Whether or not it is potent in its form, and if it is, whether or not it quickly is metabolized into other things are the important parts.
Its definitely not a common precursor to anything I've seen off of the top of my head if the structure of 3AD is what I think it is.