you have to check it in a while loop

Code:

---

action jo()
{
   cy=my;
   set(my,INVISIBLE);
   while(1){
      if(b>=4) // b is global variable and increase when molecules are join.
      {
         set(my,VISIBLE);
      }
      wait(1);
   }
}

---

PS: use the code tags to display your code