This is the 12357th most frequent German word.
viertklässler
A fourth-grade student.
Here, 'Viertklässler' refers to all students collectively in the fourth grade, used as a noun identifying the group.
Die Viertklässler haben heute einen Ausflug zum Zoo gemacht.
The fourth graders went on a trip to the zoo today.
In this instance, 'Viertklässler' identifies an individual student in the fourth grade as a representative example, used in the singular.
Ein Viertklässler sollte in der Lage sein, einfaches Lesen und Schreiben zu beherrschen.
A fourth grader should be able to master basic reading and writing.
Here, 'Viertklässler' is singular and possesses a specific context, referring to a particular student in the fourth grade recently added to the class.