This is just basic math, not cinematography. 4:3 and 1.33:1 are the same ratio of width to height, just divide 4 by 3 and you get 1.333333.... so it's the same ratio just expressed differently, 4:3 expresses it in whole numbers, 1.33 : 1 expresses it by keeping the vertical at a value of "1" and letting the horizontal not be a whole number.
This is a holdover from video versus film -- video aspect ratios are expressed in whole numbers (4:3, 16:9) and film aspect ratios are expressed with the vertical as a single unit, so 1.33 : 1, 1.85 : 1, etc.
Projection issues are a whole other thing though, you'd have to figure out how you were going to screen a 1.33 : 1 image -- probably with black borders on the sides of a 16:9 HD file or a 2K DCP.
Or you'd be making a transfer to 35mm for projection on a print, but if you have sound on the print, the actual projection aspect ratio would be 1.37 : 1 Academy and you'd have to make sure your theater could screen the Academy print format, otherwise you may have to transfer the 1.33 : 1 image with a windowbox within 1.85 : 1 print area so that a theater set-up to project only 1.85 flat or 2.40 scope could show it properly.