I'm fairly certain the RG2004 series utilizes a HD44780 compatible chipset, like the S6A0069.
While I do not have RG2004A datasheet, I believe it is very similar to the attached GDM2004D datasheet.
If correct, be aware the connector needs to be orientated at the top of the device and pin 1 is in the top left corner.
I've also attached the datasheet for the TC2004A LCD which is essentially the same LCD from a different supplier, I've utilized both these devices in past projects.
BigDog