Hey Forde,
Both forks are 41mm, so that won't be a problem, but you have a few other things to consider. I don't know if you have a single disc model with regular handlebars or a dual disc model with clip ons. All the comparisons I know are with the NA market single disc model.
- The B6 forks are shorter than the B4 forks, I forget how much, so you will change the steering geometry (faster steering).
- the stock B4 caliper has different mounting bolt spacing than the B6, so you will need to replace that
- is the rotor diameter of the B6 the same as your B4? If not you will need to change rotors.
- is the distance between the forks the same on a B6 as your B4? if not, the discs will not be in the middle of the caliper travel.
- are the dimensions of the axle the same? If not you will need a B6 axle for which you may or may not be able to get bearings to fit your wheel.
- changing just the stanchion tubes may or may not work, you really have to take them apart to see. I have had forks from 3 or 4 different Suzukis apart, and all were mutually non-interchangable, but you may have better luck.
If you decide to go through with it, let us know what you find out.
Regards,
Steve