In reply to Jerry Li.

Maybe PCA can give you a little help to decide how many clusters should be .

proc princomp data=Clusters N=2 noprint out=ScorePlot(keep= Prin1 Prin2);
var x1-x4;
run;
ods graphics / width=800px height=800px;
proc sgplot data=ScorePlot aspect=1;
scatter x=Prin1 y=Prin2 / markerattrs=(symbol=CircleFilled);
refline 0 / axis=x;
refline 0 / axis=y;
run;