Here’s the magic code for your document settings -> LaTeX preamble to get run-in headings:

```% makes 4-th level (SubSubSection) headings run-in
% page 154 of memoir manual
\setbeforesubsubsecskip{0ex}
\setsubsubsecindent{\parindent}
\setaftersubsubsecskip{-1ex}
```

What is a run-in heading? Normal headings are on their own line. Run-in headings have the text immediately following on the same line.