I checked the index, and 4-HO-aMT is hidden on page 569 of TiHKAL (under the aMT entry)... towards the top of the page:
"Markedly visual in effects, with some subjects reporting dizziness and a depressed feeling. There were, however, several toxic signs at doses of 15-20 mg orally, including abdominal pain, tachycardia, increased blood pressure, and with several people, headache and diarrhea."