  Affine varieties of dimension greater than two can be explored their
structures with the help of fibrations by the affine line or plane and quotient
morphisms by $\mathbb{G}_a$-actions. We consider $\mathbb{G}_a$-actions on
affine threefolds and discuss the structure and the singularities of the
quotient surface as well as the singular fibers of the quotient morphism.
Relative $\mathbb{G}_a$ and $\mathbb{G}_m$-actions on $\mathbb{A}^4$ or on an
affine 4-fold having $\mathbb{A}^3$-fibrations over $\mathbb{A}^1$ are
discussed when the actions leave one variable invariant or the fibration
invariant.
