There is more than the AR though, there's framing.

In 4:3 the framing is quite tight, and actors mostly fill the vertical space and the width feels somewhat constrained.

Widescreen movies are framed about the same height as 4:3, but with more space on the sides.

IMAX is filmed with the same width as widescreen movies, but with more space added to the top and bottom.

You can typically crop IMAX to widescreen without cutting off actors heads, but you cannot really do that with old 4:3 aspect shows or movies.