As Scorpio says, the "S" is used to identify models with gears, such as D2 or D2S. AFAICS I don't think there has ever been a D4 without gears though, it's always been a D4S with gears.
Fiido changed the display on many of their bikes maybe a year ago now to include a USB port, but in the process they left off the Trip and Mileage counters. Lots complained and rumour is they'll be changing the display again soon. It depends which manual you look at as to how the display is shown (older or newer). If that matters to you, check which version is on the bike before buying.