create new datatable that is made up of the four columns then insert the rows from both old table...it is so easy like this way:
DataTable dt = new DataTable("ab");
dt.Columns.Add("col2");
dt.Columns.Add("col2");
dt.Columns.Add("col3");
dt.Columns.Add("col4");
DataRow dr = dt.NewRow();
dr["col1"] = "first value in col1 from table A"
dr["col2"] = "first value in col2 from table A"
dr["col3"] = "first value in col3 from table B"
dr["col4"] = "first value in col4 from table B"
dt.Rows.Add(dr);
for sure you have to add the using loops not like that but i am giving you an example