You are correct - the Spin Off date is April 24, 2015.
On March 26, 2015 a Separation and Distribution Agreement was signed and filed with the SEC. It states in it that the completion of the Spin Off will occur on April 24, 2015.
So I too think that the 4-year Statute of Limitations ends on April 24, 2019 and not March 26, 2019.
“ The spin-off is expected to occur on April 24, 2015. The spin-off is conditioned on the satisfaction or waiver of each of the conditions set forth in the Separation Agreement as described in the Information Statement. Immediately following the distribution of CS&L shares, Windstream Holdings will effect a 1-for-6 reverse stock split. “