Differences explained from Profile Designs R&D and fitting expert David Bowden as follows
The 340 has 10mm lower rise to suit smaller arms
And the 60mm less is taken out of the shaped section
The 400 has a minimum distance from the front of the clamp of 290mm, which means that for most people the minimum length from the back of the pads to the top of the grip is 340mm - if they have the pads a long way forward. For an average rider it means 360-380 is the practical length. I (David Bowden) have mine at 375mm.
So the 340 gives smaller riders the freedom to have their pads back and still have a shaped extension