var re = from t in datatable  
         group t by new {t.a,
                         t.b,
                         t.c,
                         t.d} into g
         select new {g.key,
                     e=sum(g.e)};

------解决方案--------------------

var q =
   from p in db.Student
   group p by new {p.a,p.b,p.c} into g
   select new {
       g.Key,
       g.Sum(p => p.e)
   };