From a press release issued by FIM:

Superbike World Championship
2004 Regulations
Decision of the Superbike Commission

In a meeting held today at the circuit of Valencia, the Superbike Commission, composed of Messrs. Claude Danis (FIM), Paolo Alberto Flammini (SBK International), Takanao Tsubouchi (MSMA) and Giulio Bardi (Team representative), has unanimously approved the following proposal concerning the restrictors for 1000cc Superbike machines, for application in 2004:

The engines must be fitted with restrictors as follows:
2-cylinder: 50 millimetres
3-cylinder: 39 millimetres
5-cylinder: 27 millimetres
6-cylinder: 22.5 millimetres

All the other specifications concerning the restrictors (Art. 2.4.4.1 of the Road Racing World Championship Regulations for Superbike) remain the same.

For the 4 cylinder 1000cc engines, the restrictor rule of 32.5 millimetres has already been enforced for the 2003 season.