Look at the last SO for PS2. The game sells more in NA then Japan. If we ignore Europe for a moment (no sales data for previous SO games), these are my estimated SO4 sales:
Japan: 280K
NA: 680K
Total: 960K
White Knight Chronicles is a new IP. I think it will sell more than SO in Japan, but less in NA.
Japan: 450K
NA: 450K
Total: 900K
The two games should sell about even in Europe. So I predict SO4 to outsell WKC by about 60K.