Transform a CNF into an equivalent 3-CNF defined on the same variables
(From the comment above)
The problem seems coNP-hard; the simple reduction is from 3CNF-UNSAT (which is coNP-complete):
given a 3CNF formula $\varphi = C_1 \land ... \land C_m$, extend it adding a new clause with 4 new variables:

$$\varphi' = (y_1 \lor y_2 \lor y_3 \lor y_4) \land C_1 \land ... \land C_m$$

$\varphi'$ has an equivalent 3CNF formula defined on the same variables if and only if the original formula $\varphi$ is unsatisfiable.

($\Leftarrow$) the 3CNF formula $(y_1 \lor y_2 \lor y_3) \land (y_1 \lor y_2 \lor y_4) \land C_1 \land ... \land C_m$ is equivalent to $\varphi'$

($\Rightarrow$) suppose that $\varphi'$ has an equivalent 3CNF formula $\varphi''$ and that $\varphi$ is satisfiable.
Pick a satisfying assignment $X = \langle \dot{x}_1,...,\dot{x}_n \rangle$ of $\varphi$, and simplify both $\varphi'$ and $\varphi''$ replacing the variables $x_i$ with
the corresponding truth values $\dot{x}_i$. We get $\varphi'_X$ which is satisfiable if and only if $\varphi''_X$ is satisfiable
(both contain only variables $y_i$).
Clearly $\varphi'_X = (y_1 \lor y_2 \lor y_3 \lor y_4)$. Every clause of $\varphi''_X$ contains at most three variables,
so we can pick one of them, e.g. $(y_1 \lor \lnot y_2 \lor y_3)$, and use it to build a satisfying assignment for $\varphi'$:
$\langle y_1=false, y_2=true, y_3=false,y_4=true,\dot{x}_1,...,\dot{x}_n \rangle$ which is not a satisfying assignment for $\varphi''$,
leading to a contradiction.