Hi
When you are connecting PC3-12800 4GB RAM alone on the unit, it's giving an error message 201.
And when you are connecting the same PC3-12800 4GB RAM with 2 x 2 GB RAM PC3-10600, no error message.
But is the unit working with the same configuration and how much Available Memory it's showing?
Note : It's always best to use the same configuration RAM for best results.
And other things you can do to isolate this issue is , to check this RAM on another unit. If it's giving issue there as well, which means RAM is faulty and need to replace. Otherwise, it might be a compatibility issue.
I hope that answers your question. 
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