The 4th gen and the GX both made their debuts in late 2002 as a 2003 model. I would say the next gen for both will appear at the same time, like the Camry and ES, as well as the Land Crusier and LX.
If you're referring to the RX, I disagree. The RX300 was introduced in 1998 as a 1999 model. The second gen RX made its debut in 2003 as a 2004 model.
The first gen Highlander came out in 2001. The second gen Highlander just came out this year. The Highlander is almost a full generation behind the RX, since there's rumors saying that in 2009 or 2010, the next gen RX will be release.
As of now, both the RX and Highlander are in their second generations.