https://www.asus.com/ROG-Republic-Of-Gamers/MAXIMUS_IV_GENEZ/specifications/
Your board has 4xDIMM slots, supporting up to 32GB (4x8GB).
It's unlikely you'd notice the difference between 2100Mhz & 1600Mhz in real-world useage - there would be a difference in benchmarks, but likely nothing you'd ever notice with the motherboard & CPU.
If you try to pair 2100Mhz modules & 1600Mhz modules:
Best case: They work, but throttle the 2100Mhz to the lowest 1600Mhz
Worst case: They don't work. Some RAM just doesn't 'play nice' with each other. It's always recommended to buy a 'set' of RAM (ie. 4x8GB would max out your board - but 4x4GB is likely a substantial increase) as these sets have been tested to work together.
4x4GB G.Skill RipjawsX
http://pcpartpicker.com/part/gskill-memory-f312800cl9q16gbxl $95
4x8GB G.Skill RipjawsX
http://pcpartpicker.com/part/gskill-memory-f312800cl10q32gbxl $145
What RAM do you have at the moment?