I just switched from clap 3 to 4 and although I can see that the textual presentation is better than before, the highlighting is bold and underline rather than color. (I'm using Xubuntu 20.04 with an Xfce Xterm.)
That's not possible afaik. According to the release announcement they plan to make the colors customizable in future 4.x releases, see clap 4.0, a Rust CLI argument parser