This sounds unneccesarily complicated.
Here’s a question:
How many users and what is your total aggregate bandwidth at the main office?
Any why do you need 4 Internet connections? It looks like you are trying to build an enterprise class system on pennies, which is most likely going to come crashing down hard unless keep someone monitoring it constantly.
Also, are you sure you need to do VLAN, or is this more of a VPN question?