The number of strings on a violin, a viola, a cello, double bass, a cuatro, a typical bass guitar, and a ukulele, and the number of string pairs on a mandolin. "Four calling birds" is the gift on the fourth day of Christmas in the carol "The Twelve Days of Christmas". Groups of four
Big Four (disambiguation)
Four basic operations of arithmetic: addition, subtraction, multiplication, division. Greek classical elements (fire, air, water, earth). Four seasons: spring, summer, autumn, winter. The Four Seasons (disambiguation)
A leap year generally occurs every four years. Approximately four weeks (4 times 7 days) to a lunar month (synodic month = 29.53 days). Thus the number four is universally an integral part of primitive sacred calendars. Four weeks of Advent (and four Advent candles on the Advent wreath). Four cardinal directions: north, south, east, west. Four Temperaments: sanguine, choleric, melancholic, phlegmatic. Four Humors: blood, yellow bile, black bile, phlegm. Four Great Ancient Capitals of China. Four-corner method. Four Asian Tigers, referring to the economies of Hong Kong, Taiwan, South Korea, and Singapore
Cardinal principles. Four cardinal virtues: justice, prudence, temperance, fortitude. Four suits of playing cards: hearts, diamonds, clubs, spades. Four nations of the United Kingdom: England, Wales, Scotland, Northern Ireland. Four provinces of Ireland: Munster, Ulster, Leinster, Connacht. Four estates: politics, administration, judiciary, journalism. Especially in the expression "Fourth Estate", which means journalism. Four Corners is the only location in the United States where four states come together at a single point: Colorado, Utah, New Mexico, and Arizona.
The dosage of 1–10 mg/L is preferred as scale inhibitor, 10–50 mg/L as corrosion inhibitor, and 1000–2000 mg/L as detergent. Usually, HEDP is also used together with polycarboxylic acid (superplasticizer), in which it acts as reducing agent. Chelating agent and antioxidant
Etidronic acid is a chelating agent and may be added to bind or, to some extent, counter the effects of substances, such as calcium, iron or other metal ions, which may be discharged as a component of grey wastewater and could conceivably contaminate groundwater supplies. As a phosphonate it has corrosion inhibiting properties on unalloyed steel. Etidronic acid also acts to retard rancidification and oxidation of fatty acids. HEDP and its salts are added to detergents and other cleaning agents to prevent the effects of hard water. It is also used in peroxide bleaching to prevent degradation of peroxides by transition metals. Etidronic acid is listed as an ingredient of several cosmetic formulations where it is used for suppressing radical formation, emulsion stabiliser and viscosity control. While etidronic acid has not been limited from inclusion in cosmetics and does have legitimate uses, it is recommended that, as with most cosmetic products (particularly soaps), the product should be thoroughly rinsed from the skin after use. Etidronic acid is also included among swimming pool chemicals. It is used as a stain inhibitor to prevent metal ions coming out of solution and staining the sides of swimming pools. Etidronic acid is used in bars of soap.