I believe that your backup admin is mistaken.
According to the
OS compatibility list for NetBackup 6.x, RHEL AS 4 x64 is supported as a NetBackup master/media server, a BMR server, a BMR client, a NOM server (starting with NBU 6.5.4), a NOM-managed server, and an NDMP remote server.
Note that some of the NetBackup binaries for this platform might be 32-bit executables. There is a note in the compatiblity list that says "Next major release following NetBackup 6.5; Master server, Media servers and Clients will be supported with 64-bit binaries only. Back level support of NetBackup Media servers and Clients will be available until NetBackup 6.x reaches end of support.".
Keep in mind that you will need to purchase the NDMP licenses for NetBackup (consult with your Symantec sales rep or Symantec reseller for the correct quantity and Tier of licenses needed for your NetApp FAS3140 filers) and your backup admin will need to manually install and patch the NetBackup NDMP binaries on the master server. The NDMP binaries are not automatically installed when installing a Unix/Linux-based NetBackup master/media server (they
are automatically installed on Windows-based NetBackup master/media servers).