It would be different if it was another part of the body, but a RB can't be a RB if he can't hold onto the ball! Sorry, no tape or cast is going to help a RB hold the ball with a broken hand.
So, if they say 4-6 weeks, it probably will be at least that. If he comes back too soon, he could re-brake it or fumble a lot. They will not take any chances until he is 100%, especially with Lynch and Spiller on the team. They will take their time.
On a side note, this basically kicks down the door for Spiller to come in and be the Starter opening day. If Lynch is out 3 weeks or so, that basically means he won't touch the ball again in the Preseason. This means Spiller will get all the work with the starters and Gailey seems like the type of coach that will not play guys ahead of other guys if they don't do the work (injured or not). So, if Spiller does well, and Lynch can't get back by the 4th Preseason game (at least), expect Spiller to start if he is healthy week 1. All of a sudden Lynch and Jackson might be 2 and 3 on the depth chart instead of 1 and 2. Going to be very interesting, especially if Spiller comes out and lights it up the next few weeks. He has every chance in the world now to be the Starter opening day. Let's go Spiller (and stay healthy, by the way!!!!