Flexibility and quality improvements. See below. And you can always see full change log of Accelerator for WordPress.
- Don't optimize content for user cache.
- Fonts' option: Include to critical styles.
- Trace improvements.
- Caching wrong user context.
- Content async optimization doesn't work on sites under HTTP with CloudFlare under HTTPS.
- Content forced debug options don't apply to WordPress's lazy loading.
- Frames lazy load: about:blank treated as relative address.
- Incorrect minification of i18n.js.
- Internet Explorer 10 and lower: page is not interactive.
More information can be discovered in the original post about New version 1.4 released - Accelerator for WordPress and on the main site.