Correct, no where near as grunty, but more stable / smooth.
The B4, and I believe B5, are bringing back the low end grunt.

As for line lengths I'm pretty sure B3 / B4 are both equal length when fully powered up.

b3 lines set up should be same as b4, lines are different lengths. no support from the store?

The store tried to be really helpful. I managed to get the new kite for a grand so tried to be understanding when it appeared the shop had sold the B3 bar and lines a while back and replaced with new B4 ones. When the shop owner and I layed out a full set of B3 lines next to the B4 ones, the B3 back lines were about 200mm shorter than the B4. I assumed this would fix up the problem (when first launched it was obvious the back lines were too long). The B3 has extension lines attached to the wingtips that the B2 didn't have. As the kite chokes easily, I'm guessing I need to drop down a knot (that I've added myself to the leader lines on the bar) and accept that I can't fly it in the light winds I flew the Dos in. If I'd have known I would have bought an 11m. That said, I at least now have a good quiver of 8m Dos, 10m B3, 12m Dos.
If someone has a B3 in another size and just wants to pick up a 10m, the shop had one left about 3 weeks ago, no bar and lines though, but maybe you'd get it even cheaper than the grand I payed including bar and lines. Sunshine Coast Kitesurfing at Maroochydore. On the net or (07) 54792131. Like I said, Jonno tried to be really helpful, I honestly don't think he knew one of the boys had sold a B4 bar and lines with a B3, and they probably thought it wouldn't be an issue.

Maybe stick with B1 or B2.

Thanks for the advice guys. I'll fine tune some more. Think I may be spending more time on the 12 than I'd have hoped. If only I hadn't put my B2 10m into a tree