I like both speakers. They are about the same size and have similar sonic character. The biggest difference to me is the JBL 2226 midwoofer used in the four π is a cleaner, lower distortion model. It sounds smoother and more detailed at the same time, and is less fatiguing at high volume levels or after extended listening periods.You might also want to consider upgrading the tweeter to the B&C DE250. It can be installed in either the three π or four π models. It has smoother response.
The tweeter and crossover are basically the same in the three π and four π speakers. Depending on the tweeter used, you may have to change R1/R2. Do make sure you use the new 2008 crossover, as the improvements made around the first of the year make a difference. The changes are subtle but significant, and you can re-use many of the same components.