Toilet Drain PVC Pipe Size

I am currently in the process of moving the existing rough in toilet drain pipe in my basement. I need to move the toilet 12” to the left. I have rented a jack hammer and I have removed the concrete and gravel and I have exposed the 3" drain pipe. The pipe originally went from a 3” to 4” using 90 degree and that left a 4 “pipe exposed in the floor. When connecting a new 90 degree and a 45 do I have to go back with a 4 inch pipe to the flange or can I use all 3”. What is the purpose of the 4” section of PVC PIPE?”

4"

It is called a 4" x 3" closet bend and is the only way I ever rough in a toilet. Many use 3" pipes with no problems. So it is up to you.