I have downloaded MR scans from ADNI2 with 'Subject' IDs ###_S_4### in the CSV file, that I am trying to link to data in the diagnostic summary table. The key to link these tables is the 4 last digis 4### to the RID field in the DX table, is that correct? Are there any other common variables in the tables to verify, e.g. age?
Yes, the last four digits in the subject ID correspond to the RID. You may refer to the Roster file under Download> Study Data for a comprehensive list of all subject_id (PTID)- RID correspondences.