4th Rep
Alternatering your grip on the third rep does't nullify you doing it. That make no sense.
You make the 4th rep!
Problem With 4th Rep
The problem with the 4th rep was that you allowed it to drift forward, anway from your body. Doing so, mean you increased the weight (force) of the bar.
Think of it this way. Which is easier, performing an upright row by keeping the bar close to your body or performing a front raise.
Obviously, the close you keep the bar to you "Center of Gravity" (COG) the less force you have to produce in getting the weigh tup.
Bad Tehnique
While the 4th rep builds "Character" one of the problems is it reinforced bad technique.
As Vince Lombardi said, "Practice does NOT make perfect. Perfect practice make perfect."
Thus, one thing to keep in mind is once your form falls apart, STOP the movement. Continuing only reinforces poor techinique.
Where is your sticking point in the regular deadlift?
It hard to answer your quesition without knowing more.
Conventional Deadlift Sticking Point
Traditionally, the sticking point for Conventional Deadlifter is in the knee area. That is the "mud hole" where the bar slows down.
Now think of driving through your "Deadlift Mud hole" like driving your car though a mud hole.
You need to hit the mud hole with a running start. That running start provide you with momentum (power) which will help propel you through it.
Power = Strength X Speed
Olympic Pulls and Kettlebell Swings
The power you need to drive through that sticking point comes from the hips.
Olympic Pulls, especially Hang Power Cleans and Hang Power Snaches), are one of the most effective for increasing hip drive power.
Heavy Kettlebells Swings (100 lbs plus)
http://www.t-nation.com/free_online_...55-mcd02.hydra
Bret Contreras' has a brilliant article on this, listed above.
Kettlebell swings is movement that will increase your hip drive power.
You can make your own cheap effective "Kettlebell Swing- Adjustable-Hungarian Core Blaster" for about $20 plus weight plates.
Rack Pulls From The Knee
Again, traditionally the weak link in a conventional deadlift is the knee area.
When you pull the weight from the floor in a deadlift, you have some momentum going into the knee area. This momentum allows you to move the bar either past the sticking point or higher up before you fail.
When you place the bar in the rack in the knee area to perform Partial Rack Deadlifts, you start the pull at the weakest point with NO momentum.
That means you are sitting at a dead stop in the "mud hole". It is going to take even more strength to get out of the "mud hole"...there is NO momentum to help you.
That is why you cannot pull as much weight with Partial Rack Deadlifts from your weakest point.
NO Heal Shoe
A shoe with NO heel is best for the deadlift.
The Torque On The Power Rack Pins
One more thing, you need to move the bar up on the pins so that it is closer to the rack.
You are dropping the weight on the furtherest part of the lever arm/pin. This magnifies the force place on that lever arm.
At some point the pins at the attachment part of the rack are going to break or bend.
Think of the pins like a tree branch. The branch is more stable when it is close to the trunk/base of the tree.
The further you climb out on the branch, the greater the torque and greater the change of the branch breaking with you on it.
Kenny Croxdale