  Let $G$ be a Class 1 graph with maximum degree $4$ and let $t\geq 5$ be an
integer. We show that any proper $t$-edge coloring of $G$ can be transformed to
any proper $4$-edge coloring of $G$ using only transformations on $2$-colored
subgraphs (so-called interchanges). This settles the smallest previously
unsolved case of a well-known problem of Vizing on interchanges, posed in 1965.
Using our result we give an affirmative answer to a question of Mohar for two
classes of graphs: we show that all proper $5$-edge colorings of a Class 1
graph with maximum degree 4 are Kempe equivalent, that is, can be transformed
to each other by interchanges, and that all proper 7-edge colorings of a Class
2 graph with maximum degree 5 are Kempe equivalent.
