documentclass[a4paper,11pt]{extarticle}
begin{document}

newcommand{mystrA}{%
  a%
  or b%
  or c%
  else d%
}
newcommand{mystrB}{%
  e%
  or f%
  else g%
}
newcommand{mystr}{mystrA}

newcounter{str}

newcommand{usestr}{%
  ifcasevalue{str}mystrfi%
  addtocounter{str}{1}%
}

usestr,usestr,usestr,usestr,usestr
end{document}

ifcasevalue{str}mystrfi%

documentclass[a4paper,11pt]{extarticle}
begin{document}

newcommand{mystrA}[1]{%
 ifcase#1%
  a%
  or b%
  or c%
  else d%
 fi
}
newcommand{mystrB}[1]{%
 ifcase#1%
  e%
  or f%
  else g%
 fi
}
newcommand{mystr}{mystrA}

newcounter{str}

newcommand{usestr}{%
  mystr{value{str}}%
  addtocounter{str}{1}%
}

usestr,usestr,usestr,usestr,usestr
end{document}

documentclass{article}

newcommand{mystrA}{%
  a%
  or b%
  or c%
  else d%
}
newcommand{mystrB}{%
  e%
  or f%
  else g%
}
newcommand{mystr}{mystrA}

newcounter{str}

newcommand{usestr}{%
  ifcasenumbervalue{str}mystrfi
  addtocounter{str}{1}%
}

begin{document}

usestr,usestr,usestr,usestr,usestr

end{document}

documentclass{article}
usepackage{xparse}

ExplSyntaxOn
NewDocumentCommand{definecasesmacro}{O{str}mmm}
 {
  cs_new:Npn #2
   {
    int_case:nnF { value{#1} } { #3 } { #4 }
   }
 }
ExplSyntaxOff

newcounter{str}

definecasesmacro{mystr}
  {
   {0}{a}
   {1}{b}
   {2}{c}
   {4}{Hey!}
  }
  {d}

newcommand{usestr}{mystrstepcounter{str}}

begin{document}

usestr,usestr,usestr,usestr,usestr

end{document}