\[\boxed{\text{4.\ 393}}\]
\[\textbf{а)} - 45 + a + 31 = a - 14\]
\[\textbf{б)}\ a + 51 - 28 = a + 23\]
\[\textbf{в)}\ 4,9 - 8,9 + c = c - 4\]
\[\textbf{г)} - 0,33 + z - 0,55 = z - 0,88\]
\[\textbf{д)}\ \frac{7}{8} - 0,875 + x =\]
\[= x + \frac{7}{8} - \frac{7}{8} = x\]
\[\textbf{е)}\ p + \frac{5}{8} - \frac{3}{4} = p + \frac{5}{8} - \frac{6}{8} =\]
\[= p - \frac{1}{8}\]