OK, so as you would expect - there is an ‘official response’ here of VRRP is supported between two Peplink devices. If you do VRRP with two Peplink devices in production, Peplink support staff will provide all the assistance required to make that work.
Since VRRP is a standard it of course means that you can do HA with combinations of Peplink and non Peplink routers. I know of partners who do this all the time with CISCO routers on their customer sites where the Peplink device is the Slave for failover.
When a Peplink device is configured as a Slave, under normal circumstances the priority is 200.
So if your existing MPLS routers priority is higher than the HD4’s 200 and within the same VRRP group, in theory this should work.
eg MPLS Master priority 250, MPLS Backup priority 230, HD4 priority 200.