Just answering to my mail :

Just added :
rangeattrmap name="rmap";
range 0 - 1 / rangecolormodel=(white red darkred);
endrangeattrmap;
rangeattrvar attrmap="rmap" var=V1 attrvar=pColor;
rangeattrvar attrmap="rmap" var=V2 attrvar=pColor1;
rangeattrvar attrmap="rmap" var=V3 attrvar=pColor2;
rangeattrvar attrmap="rmap" var=V4 attrvar=pColor3;
ad then calling Pcolor* for each heatmap. It works fine !