The major advantage will be the interchangable backstraps. If a Gen 3 or Gen 3 short frame fits your hand, then go ahead and get one, but if not, then it might be worth waiting for the Gen 4.
Another consideration, I've changed my recoil springs several times for the Glock 20. Right now, I have two guide rods...one for 10mm and another for .40 S&W. Will it be easy to change out the dual guide rod as the single unit?