How to lambda the group by data on a LINQ to Sql results?

Question

1. I get the data from the database like this.

    Dim query = From t1 In TBL1 _
                Join t2 In TBL2 On t1.ID Equals t2.ID _
                Join t3 In TBL3 On t1.ID Equals t3.ID _
                Group Join t4 In t1 _
                      On t1.ID Equals t4.ID _
                      Into t4_Grp = Group _
                Select t1, t2, t3, t4_Grp
   

2. As the user performs a search I am able to filter the query results like this.

   query = query.Where(Function(o) o.t1.ID = lngID)
   

3. All works fine above. Until I want to lambda the t4_Grp. I do not know how to do a lambda expressions on the t4_Grp?