I don't see how you would need more than 1 Uno to do the job. 4 stepper motors with step/direction type drivers (like A4988 or DRV8825), 8 pins (+1 for optional motor enable). An I2C 16x2 or 20 x 4 LCD, 2 pins. A rotary encoder with push switch, 3 pins. GPS module to RX of a software serial port, 1 pin (+1 optional TX from software serial). A total of 14 (16 with options) pins of the 18 available on the Uno (leaving hardware serial for debug/upload). If you want 4 push buttons instead of an encoder, that could use only one pin using a resistor chain and 1 analog input pin.