I have used somewhere on the order of 25 grams of 4-fa in the last 3-4 months. I was using it daily for two weeks but it is an intoxicant, and it caused me to make poor choices at work and during a very important phone interview. I now only use it on weekends. Without a proper scale, it is hard for me to judge doses, but I would guess 300-500mg sipped over 8 hours(could be more, the rate at which I have consumed the weighted bags indicates more).
When I do my weekly Friday night into Saturday binge, I get a "rebound" blood pressure issue(the longer the binge the worse the effects) When I stand up I nearly faint. This can go on for 6 or more hours, and I have learned to use pilot's g-lock techniques to stay conscious. Prior to that I had to walk around with my torso lowered to keep my blood flowing where it needs to. Once I am up and acclimated, I am good until I bend over for something or sit etc. During the week, I use nothing other than caffeine and nicotine.
One more noted effect, I will get sleep paralysis bouts the second or third night after a binge. The first time scared the hell out of me, but fortunately I recalled seeing a documentary on it which calmed me down(took an hour) after what initially seemed to be one hellacious ghost/demonic encounter which is really hard for an atheist to come to grips with. With a little googling I had found that amp's and MDMA etc can induce sleep paralysis after a comedown.
I am concerned to say the least, and often google to see how many people offer similar experiences... there are only a few mentionings of fainting.
One more thing, it tears my stomach up something terrible. I get nausea the next day and a very unsettled feeling in my stomach. I have a strong constitution and nearly never vomit, 4-fa doesn't take me that far, but it is far from comfortable the next day.