1. 1.37:1 is not the same as 4:3 - that would be 1.33:1. 1.375:1 is the US academy film format (full-frame 35mm).

2. 1080p is only ever 1920x1080. Films of aspect ratios other than 16:9 (in other words, most) will be letterboxed or pillarboxed to some degree. You're not 'losing resolution' if you are retaining the full image in a format that doesn't exactly match.

3. If the image is 1080 pixels high, the width of the Wizard of Oz will be 1080 x 1.375, i.e. 1485 pixels. To answer your original question (which is only really relevant to pre-1932 films), 1080 x 1.33 is 1436 pixels wide.

4. Barely related: on Saturday I paid £20 to watch the Wizard of Oz at a drive-in. They were showing a BFI film print, but since the screen was 16:9 they enlarged the image to fill its width, thus losing the top and bottom of the picture. That's losing resolution! (Since drive-ins are a rare commodity indeed in the UK I put up with it...)