Excess fluoride in drinking water can have several negative effects on health. Let`s analyze each option to understand which one is not caused by excess fluoride.

Option 1: Deformity of teeth. Excess fluoride can lead to a condition called dental fluorosis, which causes white or brown stains and pitting on the teeth. This can result in a deformed appearance of the teeth. Therefore, excess fluoride can cause deformity of teeth.

Option 2: Hardening of bones. Fluoride helps in the development and strengthening of bones, but excessive intake can cause a condition called skeletal fluorosis. This results in the abnormal hardening and calcification of bones. Therefore, excess fluoride can cause hardening of bones.

Option 3: Stiff and painful joints. Skeletal fluorosis can also affect the joints, causing stiffness and pain. Fluoride accumulates in the bones and joints, leading to inflammation and discomfort. Therefore, excess fluoride can cause stiff and painful joints.

Option 4: Diarrhea. Excess fluoride in drinking water does not directly cause diarrhea. Diarrhea is typically caused by viral or bacterial infections, food intolerance, or other factors unrelated to fluoride intake. Therefore, the correct answer is Option 4.

In conclusion, excessive