The PS4000X is a pure SAN (it only does iSCSI over Ethernet) while the NX4 is a 'unified' system, and has the potential to replace not only your existing SAN but also your NAS/file servers as you can host shares on it. The NX4 also supports fibre channel if you ever wanted to move in that direction.
Other than the hardware differences, the management interface is where you spend 99% of your time and of the two products I would go for the Equallogic. EMC has taken its mid-90's interface (and thinking) and sqaushed it into a new system so its awkward to do stuff that should be easy.
If you could, I would go for a NetApp, one of the smaller FAS2020 or FAS2040 models. THese will gove you everything that your current 2 choices give, but also has ASIS (deduplicaton) meaning you save if you deploy virtual machines or many files with similar data (think Word and Powerpoint document metadata etc). THese saving alone can cancel out the slightly higher cost of the product, and the management interface is acutally really good.
