Thank you so much for the information, Mulleady.

I'm located in New Jersey - very close to NYC. I have no idea who in my area has the skills and experience to do the kind of modification you describe. I could ask David at Bfold, the Brompton expert from whom I bought my bike, but I don't think he's done this sort of thing, at least he never suggested it when we discussed my desire to broaden the gears. Do you think I need to have it done by someone experienced with Brompton bikes specifically, or is it only necessary that they are expert with bike mods in general? If anyone on this board knows of a tech they can recommend in my area I'd appreciate the referral.

Again, this is probably a stupid question, but do I understand correctly that I would not be removing the derailleur - that the new hub would work in conjunction with it just as the 3-gear hub does now?

Finally, I appreciate your description of the low gear being a fine hill-climber, but I'd still like to have a more accurate idea of the gears I'd wind up with before spending all this money - I have some MEAN hills by where I live! Do you have any idea how I could figure that out? I could use either the 50T or 44T front chainwheel, depending on the range of the new hub (I switched to the 44T last year but still have the 50T in my possession).