Hi,
for you as a customer the most appropriate picture is: The MPLS VPN behaves like one single IP router interconnecting your sites.
In your case just consider your 4 1721 being connected to one ISP router. There is no MPLS specific config needed on your 1721, MPLS is only within your ISP network.
This means: you send IP routing updates from one site to the "MPLS IP router simulator" and the updates will be sent further on to the other 3 1721. You forward an IP packet to the "MPLS IP router simulator" and it forwards it as IP packet to one of your other 3 1721.
If you are not dual homed or using backups then RIP would address all your needs. Also static routing might be suitable and the most simple aproach in your scenario.
Hope this helps! Please rate all posts.
Regards, Martin