hey there,
what field are you in?
and where those four unis where you have applied so far by any chance four top-notch universities?
there are many possible reasons for rejections, and many that don't have anything to do with the quality of your application. for example, the person who would ideally be supervising you from the department where you are applying can be leaving for a sabbatical, or can have too many PhD students already. or, while the department is generally good, there might just not be the relevant expertise around for supervising you.
have you been just simply going through the regular (online?) application process? then i would suggest you get in contact with the potential supervisor first, next time you apply somewhere.
good luck!