As the title states but not sure which controller to buy particularly between the 4 and 4 E set. Don't really need the ORP but would like the Salinity. Trying to establish which is the most cost-effective. If I go with the 4E my understanding is that the difference between that and the 4 is that I will lose the Salinity and ORP and S Ports but not sure that I even need the latter. What are the S Ports for? Firstly is it more cost-effective to get 4 E and add salinity through expansion card and probe? Secondly, if I do do that does the salinity card and probe go next to the Temp and PH probes and still leave me with the spare two expansion cards. The difference in cost here in UK/Europe about £170/£180