4. In a certain company, there are 100 shareholders, and any 66 of them own no less than $50 \%$ of the company's shares. What is the largest percentage of all shares that one shareholder can own?
4. Let $M$ be the shareholder owning the largest percentage of shares - $\mathrm{x} \%$ of shares. Divide the other 99 shareholders into three groups A, B, C, each with 33 shareholders. Let them own a, b, c percentages of shares, respectively. Then $2(100-x)=2(a+b+c)=(a+b)+(b+c)+(c+a) \geq 50+50+50$, i.e., $x \leq 25$. If each of the 99 shareholders, except M, owns 75/99=25/33\% of shares, then any 66 of them own exactly $50 \%$, and M owns exactly $25 \%$ of the shares.