Tabs on Top, App Tabs, Multi- touch support for Windows 7 these are new features added so far up to Firefox 4.0 Beta 3 now what next Firefox 4.0 Beta 4 to come with Firefox Sync integration and addition of tab candy features.
Firefox Sync which was been released as Weave add-on for Firefox by Mozilla after that it has been named or called as Firefox Sync which allows to sync preferences, browsing history, passwords, opened tabs across all computers.
Much like Google Chrome syncs bookmarks, preferences, extensions and Autofill with a Google account you need to have sync account or create sync account to sync browsing history, bookmarks, passwords, opened tabs on other Computers.
Now Firefox Sync has been integrated into Firefox 4.0 Beta 4 as it appears in Firefox 4.0 Beta 4 prerelease nightly build.And Tab Candy also has been added which makes tab browsing lot easier in Firefox.
Apart from that there are some minor improvements like option to removal of individual cookies from Privacy tab, you can also able to subscribe to pages without clicking RSS icon with “Subscribe to this page” provided under Bookmarks menu.
If you want to try these features you can download pre-release version of Firefox 4.0 Beta 4 nightly build, highly unstable.