You cannot monitor this using the interface state (at least not without the owners of R2 and R3 intervening, and that depends on the feature/functionality of their equipment and it's ability to have the interface state follow the MPLS VPN state... let's assume that you're not going to have much pull there...).
I assume you have some routing protocol (BGP?) running between R1 and R4? In that case I'd monitor that state to detect when the adjacency is lost.
You can monitor this with IP SLA using R1 as the source and R4 as the destination and vv