Its always good to see extensions developers making their add-ons compatible with Firefox 4 as yesterday Firefox 4 Beta 4 released Read It Later updates their Beta supporting Firefox 4 moreover Read It Later supports Panorama feature (formerly Tab Candy), so if you have installed latest Firefox 4 beta 4 then you can try beta version of Read It Later in Firefox 4.
Read It Later announced in their blog that their Read it Later for Firefox 2.07 beta now available with Firefox 4 support.
New feature in this beta release apart from support for Firefox 4 and support for Panorama is download support for sites requiring logins.Some bugs are also fixed which you can read on their blog.
To install Read it Later Beta supporting Firefox 4
1.Download Read It Later for Firefox 2.07 Beta 1
2.Save to desktop, select downloaded file through File menu>open File.
3.Install the extension and restart Firefox .
4.You can observe Read It Later button on Firefox Panorama.