When a given high-spin complex is Jahn–Teller distorted, this does not change the relative position of the total energy of the complex (assuming $\mathrm{d^{10}}$ configuration).
Indeed, the distortion only causes the degenerate triplet to split into a doublet and a singlet, and the degenerate doublet splits into two singlets.
In the case of a $\mathrm{d^4}$ high-spin complex, however, the total energy of the system is lowered, since the one ex-$\mathrm{e_g}$ electron is now in a singlet, which has a lower energy relative to the doublet.
What stops this process, which is lowering the total energy more and more? Why isn't every possible complex for a Jahn–Teller distortion automatically distorted in such a way, that we end up with a quadratic planar boundary case, as it can be observed e.g. in $\ce{[AuCl4]-}$?
