Given a locally bounded real function g, we examine
the existence of a 4-covariation $[g(B^H), B^H, B^H, B^H]$, where $B^H$
is a
fractional Brownian motion with a Hurst index $H \ge \tfrac{1}{4}$. We provide
two essential applications. First, we relate the 4-covariation to one
expression involving the derivative of local time, in the case
$H = \tfrac{1}{4}$,
generalizing an identity of Bouleau--Yor type, well known for the classical
Brownian motion. A second application is an Itô formula of Stratonovich
type for $f(B^H)$. The main difficulty comes from the fact $B^H$ has only a
finite 4-variation.