You may use 3rd party antennas on Meraki APs, but they are not officially supported. I suppose you could say they are "partially" supported. Meaning that if you open a ticket that has to do with RF coverage issues or a bridge link, and you're using 3rd party antennas, it's nothing Meraki Support can agree to formally help with. They'll likely give you a best effort or make some suggestions, but it's unofficial because of course no vendor can test and certify every model of every other vendor's antennas.
The 4x4 Cisco Aironet antenna that is fully tested and certified with the MR84 is the "Gillaroo" stadium antenna, AIR-ANT2513P4M.
Also note that if you have a use case where Meraki antennas don't suffice, when you use 3rd party antennas, it is the customer's responsibility to know what they are doing, understand EIRP and dB math, and are not violating (exceeding) any Tx power limits in their regulatory domain.