Did it smoke on tinfoil well?? If it does and your sure its number 4 heroin salts then its most likely cut with caffeine and you could possibly be more sensitive to it than the average person or the vendor went a little overboard with the caffeine ratio. Too much caffeine doesn't affect the smoking experience much but you got to smoke a whole lot more since itll be so weak and that means more caffeine is put into your body. It also wouldn't taste as good since addicts love the bitter dope taste.
I rarely smoke H so I don't have a lot of experiences to compare to, but did try it just now after you suggested it - worked pretty damn well. And I'm sure it's #4 - a sample from this batch dissolved completely and almost instantly in cold water.
I am fairly sensitive to stimulants, and coffee keeps me up, so this vendor probably was a bit overzealous in mixing in the caffeine. I'm a bit irked since I wasn't told the H had any caffeine in it, but at least I know why it keeps me up, and know to not use this late at night if I want to sleep well.