Central polynomials of minimal degree for matrices

Formanek made the conjecture that the minimal degree of the central polynomials for the $n\times n$ matrix algebra over a field of characteristic 0 is $(n^2+3n-2)\mathrm{/}2$ and this is true for $n\le 3$. For $n=4$ there are examples of central polynomials of degree $13=(4^2+3\cdot 4-2)\mathrm{/}2$ and we do not know whether there are central polynomials of lower degree. In this paper we discuss methods for searching for central polynomials of low degree and prove that the algebra of $4\times 4$ matrices does not have central polynomials in two variables of degree $\le 12$. As a byproduct of our computations we obtain that this algebra does not have also polynomial identities in two variables of degree $\le 12$.