Connecting my office branches together

I have 4 branches of my office and a HQ i want to connect it together so i can have data transferred between them, i even have a fortinet router which already has a ssl vpn connection which i can connect. And connect there specific network and use the resources, the problem is how do i connect the other branches together, so every branch can have there data accessible to all and they can even access to everyone. I really dont want to spend money on fortinet firewalls, Pfsense is a better idea to use? And connet the ssl vpn to them? Can i configure pfsense on a raspberry pi 3?