The latest LCD4bit library available from the playground uses these pin assignments:
#define RS 7 // LCD pin 4
#define RW 6 // LCD pin 5 (not used if USING_RW = false
#define Enable 5 // LCD pin 6
DB4~7 = Arduino pins 9,10,11,12
Is your LCD wired up to those arduino pins?
The LiquidCrystal code for those pin assignments assuming that pin 6 (RW) is not connected to arduino:
LiquidCrystal(7, 5, 9,10,11,12);
LiquidCrystal can tell if you want to us the RW pin or not through the number of parameters you give it. If in your case you do not want to use RW the then you would be using the form: LiquidCrystal(RS, Enable, DB4, DB5, DB6, DB7)