Stop. It's not a 4-6 defense. It was called the 46 defense because of number 46. Their box safety. He was number 46. The defense was built around him. They used base 4-3 personnel, just a unique alignment. And a super-star safety.
And yet, the 46 is largely, if not entirely dead in the league. Why? Because short, quick passes completely destroy it. In 1985, the NFL was run first, second, and third, with the occasional deep play action pass (coincidently, this is also why Montana fell in the draft. He didn't have the arm to throw regular, play action bombs). The 46 shuts this down by clogging all of the running lanes. Sounds great? Lets do that!
Well, Marino and the Dolphins happened. The 1985 Bear's one loss came at the hand of the Dolphins who were a pass-first offense. The 46 is exposed and it was just a matter of time. With the way NFL offenses work, nearly every game would end up the '85 Bears/Dolphins game.
Certain concepts survive of the 46 survive and even the Packers had a "bear" front this year: slide the ends down head up over the guards slide the OLBs in, creating a 5-man front, bring the safety down. Ta-da, it works like the 46. It was a shift, situational defense though, approximately 10 plays a game max. If you use it as your base these days, a competent quarterback (not even a good one) will carve you up with 5-7 slants all game long.