But I only have 3 persons at site to sign, can I mention 1 of the signatories as unavailable?”
Put yourself in the shoes of an auditor. What would he/she think?
Your organization, with all the time of the world, with authority, have approved a rule for documentation: SOP must have four signatures. For your organization that is the law. The auditor may think that only one signature is necessary if the person has the authority for that. However, for the auditor, the audit criteria is ISO 9001:2015 and the organization’s own rules, not his/her opinion.
So, it is not advisable to establish one internal rule and then not follow that rule. I would change the number of required signatures.