It's about 300,000 times as long as an Aspirin
In other words, 4 kilometers is 290,000 times the length of an Aspirin, and the length of an Aspirin is 0.00000340 times that amount.(for up & up aspirin, 325mg, regular strength tablets) (a.k.a. acetylsalicylic acid, a.k.a. ASA)
A single aspirin tablet measures 0.0000140 kilometers in diameter. One of the most common medicines in the world and a foundation of worldwide health systems, an estimated 40,000 metric tons of aspirin are consumed each year.
It's about 2,500,000 times as wide as a Strand of Spaghetti
In other words, the width of a Strand of Spaghetti is 0.000000420 times 4 kilometers.(average; restaurant and supermarket applications) (based on FMA Omcan Italian Made TR50 Counter Top Pasta Machine Extruder 13317)
An average strand of restaurant-grade spaghetti measures 0.00000170 kilometers in thickness. In 2010, a California Buca di Beppo set a world record by preparing a 13,780-lb bowl of spaghetti, served in a swimming pool.
It's about 3,000,000 times as long as a Grain of Sand
In other words, 4 kilometers is 3,200,000 times the length of a Grain of Sand, and the length of a Grain of Sand is 0.0000003130 times that amount.(medium grain)
Composed mainly of quartz, mica, feldspar, or magnetite, a grain of sand measures an average of 0.000001250 kilometers. In 2007, a team of seven in Myrtle Beach created the world's tallest sandcastle, which measured 0.0151 kilometers in height.
It's about 40,000,000 times as wide as a Strand of Hair
It's about 40,000,000 times as tall as a sheet of Paper
It's about 550,000,000 times as long as a Red Blood Cell
In other words, the length of a Red Blood Cell is 0.00000000190 times 4 kilometers.(a.k.a. RBCs, a.k.a. haematids, a.k.a. erythrocyte, a.k.a. erythroid cells, a.k.a. red blood corpuscles)
An average Red Blood Cell measures 7.5 µm (micrometers) in diameter. An average human has 20 - 30 trillion red blood cells in their body and each cell completes a circulatory lap in about 20 seconds.
It's about 650,000,000 times as long as a Cell's Nucleus
In other words, the length of a Cell's Nucleus is 0.00000000150 times 4 kilometers.(for animal cell)
The nucleus of an animal cell measures an average of 0.00000000600 kilometers. The nucleus is the largest organelle in these cells, accounting for about 10% of the cell's total volume.