[Samba] Problem joining Samba 4 to Samba 3 PDC

[Sonic]

I'm seeing these problems with a samba-4.5.x that I wish to be a
member of a samba-3.6.x PDC (NT4 style domain controller).
===========================
net rpc join -S PDC
...
smb_signing_good: BAD SIG: seq 1
Failed to join domain: failed to lookup DC info for domain 'EXAMPLE'
over rpc: Access denied
===========================
===========================
net rpc info
...
smb_signing_good: BAD SIG: seq 1
Could not connect to server PDC
Connection failed: NT_STATUS_ACCESS_DENIED
===========================

Please advise.

Thank you!

Chris