Four*

Four is the smallest composite number, its proper divisors being 1 and 2. Four is also a highly composite number.

Representing 1, 2 and 3 in as many lines as the number represents worked well and owes its origins to India. The Brahmin Indians simplified 4 by joining its four lines into a cross that looks like our modern plus sign. The Sunga would add a horizontal line on top of the numeral, and the Kshatrapa and Pallava evolved the numeral to a point where speed of writing was a secondary concern. The Arab's 4 still had the early concept of the cross, but for the sake of efficiency, was made in one stroke by connecting the "western" end to the "northern" end; the "eastern" end was finished off with a curve. The Europeans dropped the finishing curve and gradually made the numeral less cursive, ending up with a glyph very close to the original Brahmin cross.

Four is the atomic number of beryllium

4 dies insquequo Oris Dies!

*Thank you, Wikipedia.