The salient pole type construction of the F&P makes a much better synchronous motor than alternator. If you keep the poles long you have lots of leakage reactance, If you make them short then they are just coarse slots, with the associated cogging problem either way.
With that size beast I don't think you will have the facilities to produce an iron core. I also don't think you have the power available to justify the effort, just as I suspected you wouldn't justify using twice the number of magnets to cover the full 4" width. If you want to use some form of iron core then a stack of ring laminations without teeth round the outside of the coils would about double the flux without any cogging, but I can't imagine how you are going to produce it.
You may have a reasonable torque, but at incredibly low speed, if you insist on direct drive you will end up with a monster alternator for very low output. In this case I am sure a reasonable sized dual rotor ( free of cogging) and a timing belt speed increase is the most cost effective way.
If you must chase the direct drive route I still think you need to sort out the turbine characteristics, I think the power you have available is modest and there is no way to justify the cost of hundreds of magnets.
Flux