I won't give you the BS answer that it's a combination of the four, don't worry.
To be a pro the most important of those is DRUGS. Sucks. Shitty reality, but it's the truth. You can have great genetics, eat perfect, and train like a ****ing machine, but without the drugs you will never reach the top of pro bodybuilding. Sorry.
I can't tell you how many pros I have witnessed cheat on their diet. I have trained with some of the best pros of all time, and many train like pussies. Look at Dexter Jackson. I have never seen the man go to failure on one set, EVER. He's got superb genetics, and that along with drugs got him to the top. It wasn't his training and definitely wasn't his diet. Dude eats KFC a week out of the Olympia.
#1 Drugs
#2 Genetics
#3 Food
#4 Training
People won't like that training is #4 but if you have the top 3 training is less important. Food is the building block for muscle. No food = no muscle. Kai Greene trains like an idiot 75% of the year, but the guy can eat like it's no ones business. Don't mean Greene doesn't train hard, cause he does, but he does stupid shit till he's preparing for a show.