No, the MX does not support subinterfaces today. Depending on your design, there could be a simple method to provide a workaround, but you are still limited to 2 WAN interfaces plus a tertiary cellular backup perhaps. The workaround I mention is if you are leveraging 2 subinterfaces on a physical port today, then you simply have a dot1q trunk interface come into a switch or the MX itself, and then configure 2 access ports with the appropriate VLAN IDs (to simulate the 2 subinterfaces) and connect patch cables from those 2 LAN ports into WAN1 and WAN2. That can work around the subinterface requirement, but again, you are limited to two WAN interfaces. If you need to leverage SD-WAN functionality with 4 ISP links, today you might consider a Cisco Viptela solution.
When I hear people are using more than a couple of ISPs I ask why?
Typically if you take that some money and invest it in just two ISPs you can get a much better service.
Sometimes they are doing it for historic reasons that no longer apply.