1-testosterone. Really not a prohormone, but not as strong as real gear. Best thing you can get legally. The reason 4-AD is stacked with it is to help with some of the side effects of 1-test (there are sides, do a search).
I just started my 1-test/4-AD stack, but I'm still a little unsure on the dosage. I just upped it a bit to 150mg of 1-test (ethergel) and 900mg of 4-AD spread out over 3 doses a day... Only on the third day, though...