Why isn't 四 four lines? (the origin of 一 二 三 四 五 六 七 八 九)
This is an interesting topic; it touches one of the core idea of the Chinese language.

The Chinese language and all its dialects have not been designed by one inventor at one specific day. Instead, they were created and evolved at different regions through thousands of years at least.

Evidences (see below) showed that some of the Chinese characters from four to ten (四, 五, 六, 七, 八, 九) evolves as the type of 轉注 (轉注者，建類一首，同意相受。考、老是也。) or 假借 (假借者，本無其字，依聲託事。令、長是也。) instead of 象形 (象形者，畫成其物，隨體詰詘。日、月是也。); and one and ten (一, 十) evolves as the type of 指事 (指事者，視而可識，察而見意。上、下是也。) or 會意 (會意者，比類合宜，以見指撝。武、信是也。) instead of 象形. Any reference, say, 《鐵雲藏龜》by 清 劉鶚 in 1903 (清光緒二十九年), or the one below, can only act as possible explanations at their intellectual best.

Reference


《說文解字》by 漢 許慎.
《鐵雲藏龜》by 清 劉鶚 in 1903 (清 光緒二十九年).
《白魚解字》by 流沙河 in 2014, ISBN 9787514317329.


One Possible Explanation


一: one bar. May mean the whole, the universe. Reference.
二: two bars. May mean the positive and negative. The upper may refer to the heaven, the lower the earth. Reference.
三: three bars. The upper may refer to the heaven, the lower the earth, and the middle the human. Reference.
四: First |||| (four bars), then combines with 二 to avoid ambiguity: , like being mistaken to be two ||. Reference.
五: First a 二 with X in between: . The two bars are sky and earth, the X being the intersection between them, to mean all positive number greater than four. Reference.
六: ￼, a house with four sides plus roof and ground giving six. Reference.
七: Designed originally to be 十 (meaning all number greater than six) with a twist on the bottom: . Reference.
八: Opposite bracket-like curve  meaning "divide", used to be greatest number. See 八卦.  Reference.
九: Designed originally to be a right arm () reaching for something. Nine bears a meaning of great, the number of categories for many things. Reference.
十: | (a rope overhung) combines with a knot on it: . Using ten to mean "all" may be because of the fact that we have ten fingers. Reference.


Another possible explanation

At first, people may invent those most frequently used: one, two, and three, as put by 老子 in 「道德經」: 道立于一, 一生二, 二生三, 三生萬物. ("The Tao produced One; One produced Two; Two produced Three;
Three produced All things." translated by James Legge )