The OVA's released by Pioneer encompass OVA series 1 and 2. Series 1 ran up through part 6 with Kagato.
These new OVA's are brand new continuations. Larger plot blooming, involving that Lady Tokemi. Tenchi is supposed to hook up with someone be the end of OVA 3, and then the aftermath of everything will be covered in OVA 4. That's what the man in charge said during his early production interview.
Apparently watching GXP might help as well, because there are characters seen and others referenced to that are from GXP in the OVA as well. I haven't seen it yet though.