What 𝟒 means
It is four, in a bold mathematical form. A bold 4 in a formula may preserve a distinction in the notation, so read its role from the surrounding expression.
By 1999, mathematical and scientific publishing already used styled alphabets and digits. A Unicode proposal sought to preserve such distinctions in plain text because a character's mathematical style can carry meaning.
Should I use it outside mathematics?
Only when this Unicode character is part of the mathematical notation. For an ordinary number that just needs emphasis, format 4 in the editor. Unicode's mathematical forms are intended for notation, not as running-text styling.
The exact drawing depends on the active mathematical font; Unicode's chart does not prescribe one shape.
Don't confuse it with
1Questions people ask
1Why does 𝟒 show as a box on some devices?
Because that device's fonts have no glyph for it. The character is still correct and still copies correctly — the box is a missing glyph, not a broken character.
How to type 𝟒
𝟒\1D7D2Codes and encoding1 code points
A visible symbol can contain several code points. JS .length counts UTF-16 units; UTF-8 counts bytes.