\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage[T4, OT1]{fontenc}
\usepackage{newunicodechar}
\newunicodechar{ƒ}{{\fontencoding{T4}\selectfont\m f}}
\newunicodechar{Ƒ}{{\fontencoding{T4}\selectfont\m F}}
\begin{document}
abcdeƒgh ABCDEƑGH
\end{document}