Hello, it can only be one node after the other.
The first node migration willl produce the siteKey file for security that will be needed for other 3 nodes (that can then be migrated in parallel).
Starting from Informatica 9.6, there is concept of ‘Domain Security - Encryption key’ .This new security feature needs to be configured at the time of Informatica Installation only and requires a keyword for the new domain creation on the very first node in the domain on which you create domain. Then it will generate Sitekey which needs to be transfered to other server/nodes which you want to join in this domain.
Thus, for the first node in the domain where you upgrade it has to be done alone and you cannot start on other nodes at this time. Once installation is done on first node in domain and you have Sitekey file, then you can install/upgrade in parallel on other 3 nodes.