I would use 4.0 Beta full-time if Firebug and Yahoo's Delicious extensions were updated. The memory utilization is great just like Firefox 3.6. Compare sometime the memory utilized while running many tabs in Chrome vs. Firefox over a prolonged period of time.
Update: I found Firebug has an alpha release supporting Firefox 4. http://getfirebug.com/releases/firebug/1.7X/