Thanks for reporting this for 4.0EA1. I logged a bug titled: 4.0EA1: SQL FILE TAB IS BLANK (NO TOOLBAR, ETC) AFTER SQL DEV REOPENED
Please also note that upon reopening the product, the tab contains no toolbar, connection drop-down box, worksheet or history areas either. One workaround is to click on the Start Page tab, then click back on the SQL file tab -- it repaints with all the expected text and artifacts.
SQL Developer Team
Thanks for letting me know about the work around. I'll follow the bug for the final resolution.