= Solution
The difficulty is an unfavorable engineering scaling, rather than a fundamental diameter cutoff. For the same passive support concept and fixed allowed sag, the <elastic plate> estimate gives $t\propto D^2$. Consequently
$$
M\sim\rho D^2t\propto D^4,
\qquad t_{\rm thermal}\sim t^2/\kappa\propto D^4,
$$
where $\kappa$ is the <thermal diffusivity>. Thus increasing diameter makes the casting, annealing, transport and support progressively harder, while its long <thermal diffusion time> prevents the mirror following the changing ambient temperature. Temperature gradients deform the surface; a warm mirror also creates local turbulence that worsens <astronomical seeing>. A thinner solid disk has a shorter thermal response but is too flexible without better support.
The quoted four-metre scale is a practical rule of thumb for the simple thick passive design, not an absolute physical impossibility. The https://www.sao.ru/Doc-en/Telescopes/bta/descrip.html[official BTA description] records a 6.05-metre, 42-tonne primary already operating well before this examination. Its existence rules out interpreting the stated number as a hard bound.
\b[Mass, gravitational deformation and thermal response make straightforward thick-disk scaling impractical.]
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