MacPro mid 2010 RAM configuration

if you are NOT intending to use ANY 4 GB modules again, consider instead replacing with TWO 16 GB modules instead. This gives you 32 GB immediately, at not much more money, and a path toward higher RAM size later if needed.

it is important that you NOT try to mix 16 GB modules with smaller sizes, because they will not work together. 16 GB and larger are Registered Memory, which has a high powered output Register, which swamps the outputs of smaller modules.

RAM for this machine is Error-Correcting Code (ECC) memory, which supports fault tolerant operation, an excellent feature.

Yes.

(262097)

Modules smaller than 16 GB are almost always non-registered, and can be mixed freely with each other..

So yes, you can keep one 4GB module, provided they are as fast as the 8GB one you are getting. if not as fast, the one 4 GB will slow down all of them.