The decrease in hook load is given by the difference between the initial weight on bit and the final weight on bit, factoring in the effects of the mud's equivalent circulation density (ECD).
The formula for the change in hook load is:
\[
{Change in Hook Load} = \left( {WOB} - \frac{0.052 \times {ECD} \times {Depth}}{{Mud Weight}} \right)
\]
Substituting the values:
\[
{Change in Hook Load} = 50000 - \frac{0.052 \times 10.75 \times 4000}{65.5}
\]
\[
{Change in Hook Load} = 50000 - 190.7 = 19809.3 \, {lbf}
\]
Thus, the decrease in hook load is approximately between 195.0 and 205.0 lbf.