4. On the school playground, fifth-grade students stood in a row, one next to the other. Then, between each two students, a sixth-grade student inserted themselves. After that, between each two students in the row, a seventh-grade student sat down. Finally, between each two students in the row, an eighth-grade student arrived. At that moment, there were 193 students in the row on the playground. How many sixth-grade students are there?
First method:

Let there be $n$ fifth-grade students.

Among them, one fewer student can fit, so there were $n-1$ "sixth-graders." At that moment, there were $n+n-1=2n-1$ students of both grades on the playground.

And among them, one fewer student could fit, so there were $2n-2$ "seventh-graders."

Together, from fifth to seventh grade, there were $2n-1+2n-2=4n-3$ students.

Among them, one fewer student stood, so there were $4n-4$ "eighth-graders."

The total number of all students was $4n-4+4n-4=8n-7$.

The equation $8n-7=193$ holds.

Then $8n=200$

and $n=25$.

There were 24 sixth-grade students in the row.