Best way of chaining reduce with Java 8

If all these properties are of the same type (it seems they are all BigDecimal), you can use flatMap to create a single Stream of them all and then reduce it to the total sum:

BigDecimal total = 
    entity.getAssociate()
          .stream()
          .flatMap (a -> Stream.of(a.getPropertyA(),a.getPropertyB(),a.getPropertyC(),a.getPropertyD()))
          .reduce(BigDecimal.ZERO, BigDecimal::add);