Display maximum movement range with a non-grid-based system

I'm trying to make a video game that is similar in feel to tabletop wargames such as Warhammer 40,000 or Warmachine. In these turn-based games, there is no grid; movement and range are measured in inches or centimeters, not squares or hexes. For now, I'm using Unity's built-in NavMesh system for unit navigation. A player clicks a unit to select it then clicks the ground where she wants it to move, and the NavMesh handles pathfinding around obstacles (walls, other units, etc).

I need to limit unit movement in a turn. For example, each unit can move a maximum of 4". I want to implement a feature that displays a ring around the selected unit showing where it can move in one turn. In an area with no obstacles, this would simply be a circle of radius 4" around the unit. However, obstacles are common. How can I show the player every single point on the map that is 4" of movement away from the unit?

Below is a crude drawing of what I'm hoping to achieve. The blue dot is the selected unit, the black rectangles are walls/obstacles, and the green line is the maximum distance the unit can move in one turn.