If you use the vertical scrolling display mode, then none of that screen space is really wasted. However, with the single page display mode, you can just do a little math to compare the ratio of 4:3 against 16:10 to see how much space is left at the bottom. If we assume a 4:3 resolution of 2400 x 1800, then an equivalent 16:10 would be 2400 x 1500. Since the sheet music will be limited by the width of the device, the equivalent 4:3 aspect ratio for 1500 pixels of width is 2000 pixels of height. So you would have 400 pixels wasted, which is 1/6th of the height of the screen (16.67%). I'm sure there is probably an easier calculation than that, but that's one way to arrive at the answer. A 3:2 aspect ratio is a little better, as you'd have 2400x1600, with a page size of 2133.33 height, meaning about 267 wasted pixels, or 11% of the screen height.
As a side note, some users have mentioned the TCL Nxtpaper 14 on the forums which is around 14" in size with a 4:3 aspect ratio. The MSRP is $549, so it will be more affordable than other large tablets like the Samsung Ultra tablets or the iPad Pro. It's already available in some places in the world, and should hopefully be available in the US this year, but it definitely seems like it might be one of the best choices for reading sheet music.
Mike
As a side note, some users have mentioned the TCL Nxtpaper 14 on the forums which is around 14" in size with a 4:3 aspect ratio. The MSRP is $549, so it will be more affordable than other large tablets like the Samsung Ultra tablets or the iPad Pro. It's already available in some places in the world, and should hopefully be available in the US this year, but it definitely seems like it might be one of the best choices for reading sheet music.
Mike