have you worked on hip openers, and leg external and internal rotation exercises?  I believe the figure 4 exercise requires external rotation of that leg. if you are missing external rotation, then it will always be tight and difficult.  i also think that it is possible that your femur may not be in the right position in the hip socket when you sit.  it may drift forward and be out of socket which would make external rotation more difficult.  i just tried sitting and then the figure 4 exercise both with unadjusted sitting, and then i tried it again with my pelvis slightly moved forward, in an attempt to move the femur heads further back into the hip sockets.  this seemed to make the figure 4 a bit easier.