It will burn more power just due to the increased screen size and lighting requirements, but not a larger percentage of overall capacity. This is due to the big difference in batteries. The iPhone 4 has a 1,420mAh battery powering a screen with 614,400 pixels while the iPad has a ~6,613mAh battery and 786,432 pixels. Ignoring backlighting, the iPad has a much better power capacity to pixel ratio then the iPhone 4. Even with backlighting factored in, the iPad comes out ahead due to that massive battery.