Processor usage increases with 4GB RAM installed

Have a look at the Intel Atom® processor D2000 and N2000 series Datasheet, vol. 1.

Note pages 32-33 and table 3-24.

The takeaway from that is while your processor and memory controller support 4 GB of total RAM, they only support it in 2 GB chunks, in 2 GB per slot. Since your 412+ only has one slot, 2 GB is your max RAM. Anything above that is likely to be unpredictable.