I think the origin of the "four chunks in short-term memory" comes from giving people tasks to calculate some numbers, and seeing after how many digits they became noticeably slower.

A fact that you need to remember about code might use up more or less short-term memory in a human brain compared to a digit or a number, so don't be ashamed if your number is 3 instead of 4.

I also think that my working memory was better when I was 20ish, now at 41 I already feel less fits in and I forget it faster.