First, do you have the following in your wp-config.php file?
add_filter( 'allow_dev_auto_core_updates', '__return_true' );
If so, remove that, and you'll no longer be updated to any development builds.
Now, I typically don't recommend downgrading, you might run into unforeseen issues that no one has tested for. WordPress 4.0 is on its first release candidate, so if the version you are on includes "RC," you should be relatively safe. "Release candidate" means that the developers think it's ready for prime time, but want to get one more round of testing in just to be safe.
You're probably ok to stick with 4.0 RC until 4.0 final lands in a week or so, and 4.0 really hasn't introduced anything that would break many plugins or themes. Plus, if a plugin or theme is broken for you right now under 4.0 RC, you should contact the plugin or theme's developer, as it will likely be broken in 4.0 final too.
If you really need to downgrade, which again I stress may be more dangerous than remaining on 4.0 RC, download WordPress and delete then replace your copies of everything except the
wp-config.php file and the
/wp-content/ directory with fresh copies from the download. This will effectively replace all of your core files without damaging your content and settings. Some uploaders tend to be unreliable when overwriting files, so don't forget to delete the original files before replacing them.