Setting 4:3 Aspect Ratio

Trying to export video in 4:3 aspect ratio. It was shot in 4:3 in 4k resolution.

I export with 1920 x 1440 resolution(for 4:3) but the rendered video is not in this size.

Two screenshots are attached that show my settings and the issue in the rendered video.

When you get info on the exported file, what size is it?

I'm guessing that your input scaling setting is "scale entire image to fit." Try changing it to "Scale full frame with crop." You'll have to reframe your subject a little, but that's the price we all pay for showing pictures framed by rectangles.

I export with 1920 x 1440 resolution(for 4:3) but the rendered video is not in this size.

Likely it is. Bring it back into Resolve and check it's resolution.