How can a portal have four mods from the same agent?

As far as I know one agent can only install two mods on a portal.

But I just came across a portal that has four mods by the same agent.

How's that possible?

Solution 1:

This happens when an Enlightened agent uses a Jarvis Virus or a Resistance agent uses an ADA Refactor on a portal which already has four mods deployed; when that happens, the portal, all resonators and mods on the portal become owned by the agent who used the Virus / Refactor. (In the opposite case when an Enlightened agent uses a Refactor or a Resistance agent uses a Virus, the portal, the resonators and the mods become owned by __ADA__ or __JARVIS__ respectively).

Solution 2:

It's also possible that during some special events agents can use more mods (such as the recent anniversary event).