\documentclass{article}

\usepackage{microtype}

\newcommand\fitpar{\looseness=-1}

\begin{document}

This is a long paragraph on multiple lines, really a lot of useless lines, with
lots of useless characters and lots of useless punctuation, which could fit
better into four lines. This trick works at its best on long paragraphs because
there's more room to play with spaces and glues so we can avoid having this ugly
extra word.

\bigskip

This is a long paragraph on multiple lines, really a lot of useless lines, with
lots of useless characters and lots of useless punctuation, which could fit
better into four lines. This trick works at its best on long paragraphs because
there's more room to play with spaces and glues so we can avoid having this ugly
extra word.\fitpar

\end{document}