Hi,
Within the 4.x branch, 4.2(7d) is the latest and greatest iteration. 4.2 has been out for a while and has undergone many iterations to squash out as many bugs as we could, so I would definately recommend that one for your environments. You can find the release notes here: http://www.cisco.com/en/US/partner/docs/switches/datacenter/mds9000/sw/nx-os/release/notes/19964_11.html
However, there are two caveats in particular I would like to point out: If you are running Generation 1 Supervisors, you cannot migrate to NX-OS at all. Please check the output of the 'show hardware' command against the release notes to verify your hardware is supported on this level. Also, you will need to do an intermediate upgrade to 3.3(1c) at least or preferably 3.3(5) before going to 4.2(7d).
Please, also check with the vendors of the hardware such as storage arrays and hbas you attach to your SAN, to see if their support matrixes include 4.2(7d). The chance there is an actual incompatibility is slim, but you might end up in a situation where for example your Storage Vendor is unwilling to give you support because they have not tested their firmware with this particular version of the NX-OS.
With every firmware level, there's a corresponding Fabric Manager release. You should upgrade that first to a level that at least matches your future firmware level.
