Shattered Horizon FOV is actually defined for vertical, with the horizontal FOV defined by your aspect ratio. Here are the rough numbers calculated from that.
4:3 screens: 71
16:10 screens: 81
16:9 screens: 87
When walking on a surface
4:3 screens: 60
16:10 screens: 69
16:9 screens: 75
Shattered Horizon is a "HOR+" game
"Hor+ (Horizontal Plus)*: A hor + game is a game that when played on a widescreen monitor with a widescreen resolution, expands the horizontal component of the FOV while keeping the vertical component roughly or exactly the same. This is often considered the ideal solution for widescreen games, as it grants widescreen users a wider picture."
This means that you can get a wider FOV on 4:3 screens by playing in a windowed mode and setting a widescreen resolution (1280x720 for example).
If you think that the FOV is still narrow even like this, feel free to comment further.