20GB ram mac pro

What model Mac Pro?

Assuming a 2103 model:

You can't mix UDIMM and RDIMM modules. Thus you can't use a 16 GB module with either a 4 or 8 GB module.

Yes, you can add an 8GB module to your existing three 4 GB modules

http://blog.macsales.com/22745-mix-and-match-more-memory-faster-mac-pro-2

The memory slots in the late 2013 Mac Pro dark cylinder can be upgraded by the each. Matched pairs are NOT required, but if you accidentally end up with matched pair in adjacent slots, they will "buddy up" for a very slight speed improvement (not really noticeable in daily use).

I want to re-emphasize what lllaass said above:

If you install ANY Registered DIMMs (typically 16Gb or larger) your Mac will not start until you remove ALL the ordinary un-Registered DIMMs. They do not play nice together.

The upshot of this feature is that if you ever expect to go really large on the memory, you have to do so with Registered DIMMs and set aside your Un-registered DIMMs. So in that case skip the smaller DIMMs and start your Big Memory Upgrade with the Registered DIMMs.