macro myfunction,arg1,arg2,arg3,arg4 
{
     if argc=3
          push arg1
          push arg2
          push arg3
          call function1
     else if argc=4
          push arg1
          push arg2
          push arg3
          push arg4
          call function2
     end if
}    

if arg4 eq
  ; three args
else
  ; four args
end if    

macro myfunction,[arg]
{
common
     local count
     count = 0
forward
     count = count + 1
     push arg
common
     if count = 3
          call function1
     else if count = 4
          call function2
     end if
}
    

macro mac [arg]
{
  argc = 0
forward
  if ~arg eq
    argc = argc+1
  end if
common
  ; now argc contains number of macro parameters
  (...)
}