The double space generally works well. However, sometimes the lacking newline in the PDF still occurs to me when using four pound sign sub titles #### in Jupyter Notebook, as the next paragraph is put into the subtitle as a single paragraph. No amount of double spaces and returns fixed this, until I created a notebook copy 'v. PDF' and started using a single backslash '\' which also indents the next paragraph nicely:
#### 1.1 My Subtitle \
1.1 My Subtitle
Next paragraph text.
An alternative to this, is to upgrade the level of your four # titles to three # titles, etc. up the title chain, which will remove the next paragraph indent and format the indent of the title itself (#### My Subtitle ---> ### My Subtitle).
### My Subtitle
1.1 My Subtitle
Next paragraph text.