Hi RB,
Yes it's possible to upgrade from SAP ERP 2004 to EHP7.For more information on this SAP recommended two SAP Notes 1737650 - EHP7 for SAP ERP 6.0 SP Stacks - Release & Information Note& 1742750 - Upgrade from SAP ERP 2004 to EHP7 FOR SAP ERP 6.0 (Java) .
Recently i'm performing this upgrade & following these two SAP Notes.
Regards,
Gaurav