The four points $A(-1,2), B(3,-4), C(5,-6),$ and $D(-2,8)$ lie in the coordinate plane. Compute the minimum possible value of $PA + PB + PC + PD$ over all points P .
1. **Identify the coordinates of points \(A\), \(B\), \(C\), and \(D\):**
   \[
   A(-1, 2), \quad B(3, -4), \quad C(5, -6), \quad D(-2, 8)
   \]

2. **Find the equations of the lines \(AC\) and \(BD\):**
   - For line \(AC\):
     \[
     \text{slope of } AC = \frac{-6 - 2}{5 - (-1)} = \frac{-8}{6} = -\frac{4}{3}
     \]
     Using point-slope form with point \(A(-1, 2)\):
     \[
     y - 2 = -\frac{4}{3}(x + 1)
     \]
     Simplifying:
     \[
     y - 2 = -\frac{4}{3}x - \frac{4}{3}
     \]
     \[
     y = -\frac{4}{3}x + \frac{2}{3}
     \]

   - For line \(BD\):
     \[
     \text{slope of } BD = \frac{8 - (-4)}{-2 - 3} = \frac{12}{-5} = -\frac{12}{5}
     \]
     Using point-slope form with point \(B(3, -4)\):
     \[
     y + 4 = -\frac{12}{5}(x - 3)
     \]
     Simplifying:
     \[
     y + 4 = -\frac{12}{5}x + \frac{36}{5}
     \]
     \[
     y = -\frac{12}{5}x + \frac{36}{5} - 4
     \]
     \[
     y = -\frac{12}{5}x + \frac{16}{5}
     \]

3. **Find the intersection point \(P\) of lines \(AC\) and \(BD\):**
   \[
   -\frac{4}{3}x + \frac{2}{3} = -\frac{12}{5}x + \frac{16}{5}
   \]
   Clear the fractions by multiplying through by 15:
   \[
   -20x + 10 = -36x + 48
   \]
   Solving for \(x\):
   \[
   16x = 38
   \]
   \[
   x = \frac{38}{16} = \frac{19}{8}
   \]
   Substitute \(x = \frac{19}{8}\) back into the equation of line \(AC\):
   \[
   y = -\frac{4}{3} \left( \frac{19}{8} \right) + \frac{2}{3}
   \]
   \[
   y = -\frac{76}{24} + \frac{2}{3}
   \]
   \[
   y = -\frac{19}{6} + \frac{4}{6}
   \]
   \[
   y = -\frac{15}{6} = -\frac{5}{2}
   \]
   Thus, the intersection point \(P\) is:
   \[
   P \left( \frac{19}{8}, -\frac{5}{2} \right)
   \]

4. **Calculate the distances \(PA\), \(PB\), \(PC\), and \(PD\):**
   - Distance \(PA\):
     \[
     PA = \sqrt{\left( \frac{19}{8} + 1 \right)^2 + \left( -\frac{5}{2} - 2 \right)^2}
     \]
     \[
     PA = \sqrt{\left( \frac{19}{8} + \frac{8}{8} \right)^2 + \left( -\frac{5}{2} - \frac{4}{2} \right)^2}
     \]
     \[
     PA = \sqrt{\left( \frac{27}{8} \right)^2 + \left( -\frac{9}{2} \right)^2}
     \]
     \[
     PA = \sqrt{\frac{729}{64} + \frac{81}{4}}
     \]
     \[
     PA = \sqrt{\frac{729}{64} + \frac{1296}{64}}
     \]
     \[
     PA = \sqrt{\frac{2025}{64}} = \frac{45}{8}
     \]

   - Distance \(PB\):
     \[
     PB = \sqrt{\left( \frac{19}{8} - 3 \right)^2 + \left( -\frac{5}{2} + 4 \right)^2}
     \]
     \[
     PB = \sqrt{\left( \frac{19}{8} - \frac{24}{8} \right)^2 + \left( -\frac{5}{2} + \frac{8}{2} \right)^2}
     \]
     \[
     PB = \sqrt{\left( -\frac{5}{8} \right)^2 + \left( \frac{3}{2} \right)^2}
     \]
     \[
     PB = \sqrt{\frac{25}{64} + \frac{9}{4}}
     \]
     \[
     PB = \sqrt{\frac{25}{64} + \frac{144}{64}}
     \]
     \[
     PB = \sqrt{\frac{169}{64}} = \frac{13}{8}
     \]

   - Distance \(PC\):
     \[
     PC = \sqrt{\left( \frac{19}{8} - 5 \right)^2 + \left( -\frac{5}{2} + 6 \right)^2}
     \]
     \[
     PC = \sqrt{\left( \frac{19}{8} - \frac{40}{8} \right)^2 + \left( -\frac{5}{2} + \frac{12}{2} \right)^2}
     \]
     \[
     PC = \sqrt{\left( -\frac{21}{8} \right)^2 + \left( \frac{7}{2} \right)^2}
     \]
     \[
     PC = \sqrt{\frac{441}{64} + \frac{49}{4}}
     \]
     \[
     PC = \sqrt{\frac{441}{64} + \frac{784}{64}}
     \]
     \[
     PC = \sqrt{\frac{1225}{64}} = \frac{35}{8}
     \]

   - Distance \(PD\):
     \[
     PD = \sqrt{\left( \frac{19}{8} + 2 \right)^2 + \left( -\frac{5}{2} - 8 \right)^2}
     \]
     \[
     PD = \sqrt{\left( \frac{19}{8} + \frac{16}{8} \right)^2 + \left( -\frac{5}{2} - \frac{16}{2} \right)^2}
     \]
     \[
     PD = \sqrt{\left( \frac{35}{8} \right)^2 + \left( -\frac{21}{2} \right)^2}
     \]
     \[
     PD = \sqrt{\frac{1225}{64} + \frac{441}{4}}
     \]
     \[
     PD = \sqrt{\frac{1225}{64} + \frac{7056}{64}}
     \]
     \[
     PD = \sqrt{\frac{8281}{64}} = \frac{91}{8}
     \]

5. **Sum the distances \(PA + PB + PC + PD\):**
   \[
   PA + PB + PC + PD = \frac{45}{8} + \frac{13}{8} + \frac{35}{8} + \frac{91}{8}
   \]
   \[
   PA + PB + PC + PD = \frac{45 + 13 + 35 + 91}{8}
   \]
   \[
   PA + PB + PC + PD = \frac{184}{8} = 23
   \]

The final answer is \(\boxed{23}\).