4th Suit Forcing
One source says:
...there is practically no bridge player who plays [the fourth suit bid at responder's second turn] as non-forcing, and therefore this term has become non-descriptive.
However, Bridge Conventions Complete explains that Fourth Suit Forcing is forcing to game. In standard bidding, it is only forcing for one round.
For more along these lines, see
Ron Klinger's notes. (If this link is bad or you are not on the web, use this one.)