Welcome to the gShield project (aka grblShield). If you just landed here and are not sure what a gShield is here is a quick summary:
The Arduino gShield is a complete hardware solution the grbl CNC motion control software. gShield/grbl is compatible with the Uno and other 328p versions of the Arduino development platform.
The gShield can also be used to run TinyG2 on the Arduino Due. TinyG2 is a ARM port of the TinyG motion control system.
The currently shipping version of the gShield is v5. If you are looking for documentation on the gShield be sure to checkout the gShield wiki.
Differences between gShield v4 and v5
Other than price, there is almost no difference between v4 and v5. The v5 has added a power selection section that allows you to select if you want to run the board from 3.3 volts, 5 volts, or the voltage selected by the Arduino (IOREF). This was added to better support the Arduino Due and TinyG2. You can find out more about v5 here