Answer
Although there is no inherent limit to the total number of labels that a given packet can support, the number of labels that a router can push/pop is limited.
Four-label push/pop is not supported for MPLS for Juniper Networks devices. Support for four-label push/pop would only be needed for VPN PE routers that could be the ingress for an RSVP LSP, as well as for an LDP LSP that tunnels over the RSVP, and then egresses at a router beyond the endpoint of the RSVP LSP.
For Juniper Networks devices, the VPN PE routers only need to push two labels (VPN and LDP) and the core routers only need to swap and then push up to two labels (swap LDP, push RSVP, and bypass). This provides LDP tunneling in RSVP LSPs, along with link protection.
For more information, click Layer 3 VPNs Feature Guide.