Its busted due to a change in how SQL Developer loads components. BRIDGE being a migration component, it doesn't load until a migration action is performed (like Copy to Oracle or opening the Translation Scratch Editor).
Quick workaround: Start SQL Developer, Tools > Migration > Scratch Editor. Now when you open a worksheet the BRIDGE command will be recognized.
Note I think there was some work recently on BRIDGE so SQL Dev 4 EA2 should be used.
Ill work making BRIDGE work without having to do anything special.