What is an empty four-character idiom?

Source:

1, the blue sky in Wan Li is like washing, the cold is soaking in the bright moon, and the curtain is rolling with jade waves.

At that time, the sky was blue, and a flock of geese descended from the sky and flew in a herringbone shape.

Selected from: Song Yuangan's "Water Tune Song Tou" and the essay "Long River Autumn (Dynamic and Static)".

Interpretation: refers to the blue sky, such as washing, and describes the sunny weather.

Example:

1, the bright moon is in the sky, the blue sky is like washing, and the air is mixed with faint osmanthus fragrance, which is refreshing.

2, the joy of first meeting, long-lost sigh, all with a cup of warm sweet tea, in a sunny afternoon, such as washing the blue sky.

3, open the slate, suddenly suddenly enlightened, the blue sky outside, rustling reeds into my ears.

The blue sky is like a sentence.

1, the snowy mountains on the riverside are boundless and the blue sky is like washing. Looking around, the sky is shrouded in a thick fog, gloomy and mysterious.

2, spotless streets, green trees, flowers, blue sky, such as washing, clear Wan Li, calm.

The sky is blue in the morning. Suddenly, there was a deafening noise overhead, which attracted everyone's attention on the earth.

When the rain clears and the blue sky is clear, the rainbow appears on the horizon, and its soft body stretches like a gorgeous ribbon.

It's nine o'clock in the morning, the blue sky is like washing, the sun is shining, and the mountains are like mountains. Fan Jingshan is full of pine wind and blue.