It looks like you are thinking about the wire diameter.
Two pieces of #10 would fit side by side, in the same hole as a piece of #4.
The empty space is not carrying current, so that area cannot be included.

"Compare the circular mill area."
The circular mils is the part that carries the current.  That's what is important.
The circular mils of #10 is about 10,000.
Two pieces of #10 will have about 20,000 circular mills, and that's about #7.

Four pieces of #10...
Two #10 is #10-3=#7, and two #7 is #7-3=#4.  
It holds up as a quick rule of thumb.  #4 is about 40,000 circular mills, same as 4 x the 10,000 of #10.