Question 1183252
Draw this:
This is an obtuse triangle with the first leg (west) 4 km
The second leg is unknown but the angle is 107 degrees, 90 for a track of 180 and 107 deg for a track of 197 deg.
The angle between where he started and the route back is 45 degrees, since he is southwest of where he started.  
Use the law of Sines
sin 107/long side (answer)=sin 28/4 km
so long side=4*sin 107/sin 28
=8.15 km