The LCD panel in your X900E cannot display 4 x 3840x2160p, nor is the Hopper3 capable of displaying that. There isn't even an HDMI standard for that resolution. 3840x2160p is actually the same total resolution as 1920x1080p x 4. You can check the math yourself:
3840 x 2160 = 8294400
1920 x 1080 x 4 = 8294400
Each 1080p screen is 1920x1080. When arranged in a 2 x 2 rectangle, like Multi-view, they fit in a 3840x2160 space. Think of it more as 1920 x 2 x 1080 X 2 as you are only doubling the length and width, not quadrupling them. I hope that helps.
FWIW: I believe you that they look better in Multi-view on your set. I suspect Sony is just doing something clever that we don't understand.