If you are familiar with WordPress codes, then find the hooks used on ‘header.php’ file and override the hooks from the Child theme. Our theme is child theme compatible so that you can change the hooks from the child theme easily. ‘get_header()’ function is used to call header.php file in WordPress. You can use the conditional programming on this case, check that page is 404 page or not, if that page is 404 hide menu.
I think you can do it easily. You can find the hooks insides the acmethemes folder.