For any real number $$t$$, let $$\lfloor t\rfloor$$ denote the largest integer $$\leq t$$. Suppose that $$N$$ is the greatest integer such that $$\left\lfloor\sqrt{\left\lfloor\sqrt{\left\lfloor\sqrt{N}\right\rfloor}\right\rfloor}\right\rfloor=4$$. Find the sum of digits of $$N$$.