Try
SELECT table1.Opp_Id, table1.Objective, table1.Revenue, table1.Name_Type, table4.QuoteNo,
case when table2.Name is null then table1.Name else table2.Name end as CustomerName
FROM ((table1 LEFT OUTER JOIN table3 ON table1.Opp_Id=table3.Opp_Id)
LEFT OUTER JOIN table4 ON table1.Opp_Id= table4.Client_Id)
FULL OUTER JOIN table2 ON table3.Client_Id=table2.Client_Id
Not sure what your database is but there are functions that could be used instead of the Case statement
Oracle
NVL(table2.Name, table1.Name)
SQL server
isnull(table2.Name, table1.Name)
Ian