Four spaces in front of a paragraph creates a code block. However, three spaces nicely formats a paragraph, putting it in line with bulleted or blockquoted paragraphs. That is, it does in Edit mode. But in Preview mode, the indentation vanishes, leaving the paragraph aligned to the left.
This is because Markdown eliminates white space like this when it’s rendered. The best way to achieve the look you want is probably via style settings—i.e., CSS, or by installing a theme that gives you first-line-indent on paragraphs.
I suspect that the easiest answer might be to write in bullets, using CSS to hide the bullets. (I think there’s a native hotkey toggle for it too, but only it hides indents in Live Preview, not Reading View.)