4-HO-DMT, which stands for 4-hydroxy-n,n-dimethyltryptamine, is the chemical name for the drug psilocin, which is found in many species of psychedelic ("magic") mushrooms. 4-HO-DMT has a very high safety profile, there are no known overdoses that I know of (you would have to take SO MUCH).
The drugs responsible for the main psychedelic effects of magic mushrooms are psilocin and psilocybin. Psilocybin, which is 4-PO-DMT, converts into psilocin in the brain after ingestion because the molecule is too big to pass through the blood brain barrier.
Pure psilocin is sometimes obtained by synthesizing it in its pure form or extracting it from mushrooms. This is very rare and you are much much more likely to stumble upon mushrooms. An average eighth (3.5g) of mushrooms contains around 15mg of pure psilocin. So you can figure out the dose from that.
The experience usually lasts 4-7 hours total (shorter than LSD), and is characterized by laughing a lot and feeling very relaxed. There are open and closed eye visuals, pupil dilation, and a general "mindfuck" aspect to it. It is sometimes referred to as orally active DMT or long-lasting DMT.
There is always a possibility of a bad trip on 4-HO-DMT/mushrooms, so set, setting, and mindset are very important if you want to have a good trip. Just remember, no matter how "sure" you are that the trip will never end and that you are dying, it is not true. Everyone comes back.
I just took 30mg of pure 4-HO-DMT hydrochloride. That's the equivalent of around 7 grams of psilocin/psilocybin-containing mushrooms. I'm going to be incapacitated for the next 8 hours or so, so don't bug me or try to freak me out.