Can I use a pin in a 4-pin LCD group for non-LCD functionality, such as a general purpose input?
Yes, it is possible to use a pin in a 4-pin LCD group for non-LCD functionality. See the LCD_SEGEN register definition in the reference manual: "Determines which segment lines are enabled. Each bit represents a group of 4 segment lines. To enable segment lines X to X+3 [...] Each LCD segment pin can also be individually disabled by setting the pin to any other state than DISABLED in the GPIO pin configuration."