Please note that there are subtle differences between DDR3 and DDR4 modules.
Key notch difference
The key notch on a DDR4 module is in a different location from the key notch on a DDR3 module. Both notches are located on the insertion edge but the notch location on the DDR4 is slightly different, to prevent it the module from being installed into an incompatible board or platform.
DDR4 modules are slightly thicker than DDR3, to accommodate more signal layers.
DDR4 modules feature a curved edge to help with insertion and alleviate stress on the PCB during memory installation.