el cuarto grado
You could say "el quinto grado".
Estoy en el tercero grado (I am in the 3rd grade)
It depends on what you mean by "grade" If you are referring to quality, such as "what grade is this cut of meat?" or "I don't care what grade you buy as long as it does the job." The Spanish for "what grade" is "Qué grado?" If you are referring to an academic grade describing how many points you have on an exam or similar, the Spanish for "what grade" would be "Cuál nota?"
cuatro
Los cuatro.
Cuádruplex
You can say "Ella era en el sexto grado."
You say "cuatrocientos libros".
estoy enfadado/enfadada (girl)
estoy en el grado 5
you mean how to say dog in Spanish? well, it is "PERRO" in Spanish. I hope that helps.
You can say "Tres o cuatro".