What is the ruling on someone who washed the parts of wudu three times, then found a part that water had not reached, so he completed his washing for the fourth time? Is his action valid, or must he repeat the prayer?

A washing is not counted until water covers the entire limb. If the coverage is not achieved, that washing is nullified. Washing more than three times is valid, though disliked, and does not invalidate the ablution. This is the view of the majority of scholars. Based on this, the ablution of the person you asked about is valid, and he is not required to repeat either the ablution or the prayer.