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Written solution:

Grandfather spent $\frac{2}{5}$ hours jogging on Monday. He spent $\frac{2}{3}$ hours walking on Wednesday and $\frac{2}{4}=\frac{1}{2}$ hours on Friday. Converting these times to minutes, we get $$\dfrac{2}{5} \cdot 60 = 24~\mathrm{minutes}$$ $$\dfrac{2}{3} \cdot 60 = 40~\mathrm{ minutes}$$ $$\dfrac{1}{2} \cdot 60 = 30~\mathrm{ minutes}$$ for Monday, Wednesday, and Friday respectively. Therefore, Grandfather totaled $94$ minutes of exercise throughout the week.

If he walked at a pace of $4$ miles per hour each day, he would have spent $\frac{2}{4}=\frac{1}{2}$ hours each day walking. This equals $\frac{2}{4} \cdot 60 = 30$ minutes every day. If he did this for $3$ days, he would have totaled $3 \cdot 30 = 90$ minutes of exercise.

Therefore, Grandfather would have walked for $94 - 90 = 4$ less minutes.

Thus, D is the correct answer.