A rod which has a diameter up to "4 inches", will be easily undergoes to a 4 inch pocket for the purpose of hanging curtains, drapes, valance, and backdrops.
4 inch rod pocket is best suited for valances, door curtains and window curtains. 4 inch rod pocket is the one of the most used patterns for curtains hanging.
Rod which has diameter is less than 4 inches, will also be a perfect fit for curtain hanging.
This is not best suited for long width backdrops, which are above than 8 feet (96 inches)
Sometimes it gets hard to glide long width curtains and drapes using 4 inch rod pocket.
For long width curtains grommet is better option for smooth sliding of curtains both side (Left and Right). Also Traverse rod is better option for the same.