I noticed the z-axis is driven through gear with a single motor. Can the design driven via 2 motors with threaded rod attached via coupler, like most reprap printers?
Also, the bed is supported via one or both sides with the z rods? I have also seen single and double smooth rods on each side for the Z-axis.
What are the advantages and disadvantages of each design?
4 smooth rods are just gonna give u overkill stability. If that sounds good to you then go with the 4 rods 
the xmaker is the design with the z leadscrews, I think all others have belts, but it is also the one with double rods. The leadscrews / belts are attached on both sides of the z stage to move it smoothly.
You can hook up the leadscrews or belts to two steppers just like a Mendel design, same wiring, in the end this is still a reprap and the electronics could be used on or even taken from a Mendel design.
I like the idea of twin steppers on the z stage, to me it means that the stage is more likely to be driven from both sides simultaneously, whereas a belt driven design could only have more slop (although this may be undetectable).
But I have had issues on my Mendel where one stepper has stalled and the other hasn’t, twisting the stage out of square and would probably break something if I weren’t watching it… The leadscrews should be MUCH smoother than my m5 rods though so it may never happen in this design but… there’s a few pros and cons for ya 
Do not use 4 smooth rods, it’ll be severely overconstrained. Use only 2 rods and single threaded rod. Otherwise, and misalignment, including inevitable kinks and imperfections in threaded rod, will cause the z axis to bind
They’re ballscrews - no reason to have only one - and the 4 posts is working fine, its possibly not necessary, but it hasn’t caused binding…
I am using 8mm rod and leadscrew that’s why I am thinking of 4 rods.
I will use 2 motors as well.
I don’t see any problems there