x1=p4 is the locus/place of (vocal) articulation of sound/vocal product x2=v1=c2 made by instrument/locus/vocalization tool x3=p3 in individual speaker x4=v2=c1=p1
x3 will most likely be loci of the surface of the tongue, lips, teeth, interior regions of the cheeks, uvula, epiglottis, glottis, pharynx, roof of the mouth, jaws, etc. (Arguably could apply to regions of crickets legs aand other parts of the body for various organisms/entities, including humans; but it must be "vocal" in a generalized sense).