Personally, I would start learning .NET 4 as it's the most recent version and has new and better features and fixes than 3.5. Besides the chapter on jQuery, the new book has a chapter on the ADO.NET Entity Framework replacing the one on LINQ to SQL.
However, if you already have the 3.5 book, you could also decide to get Professional ASP.NET 4 instead and use the two side by side. Most of the stuff discussed in the 3.5 version still works in VWD 2010.
But if you're really brand new to .NET, never did a lot with the 3.5 version and think the Pro edition is a bit too much, I would start with Beginning ASP.NET 4.