D means not as well spec'd as P. 4 speed SA hubs at least the English ones were less reliable and needed more careful adjusting. I had a 7 speed Shimano and it was reliable but heavy and less efficient than 3 speeds.

[QUOTE=Bonzo Banana;20287110]https://dahon.com/bikes/curl-i4/

Curl i4 with a 3 speed hub?! I thought the i stood for the number of internal gears same as D7 and D8 for derailleurs etc. Some inconsistencies on that page. Does mention 4 gears further down. I think for the small gain of a fourth gear I'd prefer the greater reliability of the 3 speed hub. Maybe I'm wrong but I seem to remember the 4 speed version was noticeably less reliable than the 3 speed and internally much more complicated and shared more of its design with 5-7 speed hubs. Considering they have the i8 version for those who need wider gear ratios my preference would be a i3 version anyway in the range for those lighter and more fit that just want more reliability for maybe an everyday commuter bike.