Can Redouble More Than Once
The wizard's strategy assumes the typical redouble rules, which are that you can redouble more than once, usually twice for a total of 3 doubles. You would end up with 8 units out if you doubled 3 times. So the EV of redoubling 6 vs 4 includes the possibility of redoubling again if you end up with 8, 9, 10 11. Also included is the benefit of possibly surrendering (rescue) after any subsequent double.
From my own software, the EVs of the available actions after doubling once out of possibly 3 times and ending up with 6 vs 4 are:
Double: -0.2295,
Stand: -0.2816,
Surrender: -0.5000
So doubling again is actually a defensive play in this case. Also note that, once you have doubled the max number of times, you use the normal rescue strategy after the final double, assuming that they take half of your total wager.
Gorgon is also correct that it is impossible for some of these plays to occur if you follow the basic strategy. This is one of them. The only way you can double and end up with 6 vs 4 is if you doubled a pair of 2's and received another 2. However the basic strategy says to split 2,2 vs 4. So you would only consider this on the last hand of a 4 way split. The Wizard's strategy doesn't even include an entry for 4 vs 4 but, once again, using my own software, I can tell you that you should hit 2,2 vs 4 when you can't split it. So 6 doubled once vs 4 should never occur. It is calculated as part of the CA analysis and I suppose it might be useful to know what to do if you made an error and doubled that last 2,2 vs 4.
