Yes it will work behind a fortigate firewall so long as your forward the right ports from the outside world through to the device hosted in the office.
As to the question of which device to use. That depends on required throughput and the number of remote devices you plan to have in the future. If it will only ever be a single HD4 in the field (with a max of two), and since you already have the fortigate doing the perimeter routing so you’re just looking for a VPN endpoint you might consider a Balance One (30Mbps of VPN throughput) or even a BR1 (20Mbps of VPN throughput).
Set up correctly, if you used a BR1, you could provide LTE failover to your office network as well as provide VPN connectivity to the remote HD4 which is a nice value add.