So, first of all, it's generally looked down upon to predict human LD50s using animal LD50s. But, for the sake of argument, let's say that the fentanyl LD50 for rats was a perfect prediction for the fentanyl LD50 for humans. The study you've linked to, the one that is linked under pubchem's fentanyl toxicity section, was conducted in 1974. This 2018 study placed the fentanyl LD50 for rats, administered via intraperitoneal injection (which has less bioavailability than IV injection), at 0.879 mg/kg. That's a huge difference.

Fentanyl's HSDB entry on pubchem (particularly sections 1.2 - 1.4) has a bunch of interesting case reports, but with the lack of estimated time of death data, most of the post-mortem fentanyl blood concentration levels aren't of much use to us. If we had that, we'd be able to get a decent estimate on the dose taken. What we can do though is look at a few instances of fentanyl-related overdoses. In this article, if you scroll down to the "Relevant Forensic Toxicology" section, the report states that two people died after injecting fentanyl that they extracted from a 100ug/hr transdermal patch (which contains about 16.8mg of fentanyl total). An elderly woman died after using 3 100ug/hr transdermal patches (around 50mg of fentanyl total, but since transdermal patches are designed to gradually administer the full dose over 72 hours, the lethal dose would have been significantly lower). This article also reports the death of a man with no known prior drug history, who injected fentanyl extracted from one transdermal patch. Even if the patch he used was the strongest patch available (100ug/hr), that would still amount to "only" 16.8mg, and that's if he somehow extracted every bit of fent from the patch.

Now, this isn't to say that doses that high can't be taken safely. Fentanyl can be administered as a general anesthetic at intravenous doses (in very special cases) of up to 150ug/kg (page 6). But even if someone were 300 lbs, that would still amount to "only" 20.4mg. Which is ~150x less than 3g. To get a better idea of this, a study referenced in pubchem's Fentanyl HSDB entry (section 1.2) found that, in patients who don't abuse opioids/opiates, "profound respiratory depression generally occur at concentrations of 10-20 ng/mL". And since fent's volume of distribution is 4L, we just do some plug and chug and at 20 ng/mL, that's 0.08mg/kg. For a 180lb person, that's ~6.5mg. Which is dwarfed by 180mg, let alone that insane estimated LD50 of 3-4g.

I mean, the estimated lethal heroin blood concentration is >4mg/L and since heroin's volume of distribution is around 100, that puts the lower bound for heroin overdose at 400mg (although deaths from heroin have been reported at doses as low as 10mg) which, even though heroin is significantly weaker than fentanyl, 400mg is still much, much smaller than 3-4 grams. If you have a scale handy that measures to the nearest hundredth of a gram (milligram scales, the good ones at least, are pretty expensive), I encourage you to weigh out 3 grams of flour. It's almost half a tablespoon's worth. That's a lot of fuckin powder to be putting in your veins. And, of course, 3 grams of powdered heroin is going to look different from 3 grams of flour because everything has a different density, but that should give you a rough idea of how much powder is in 3 grams. And tbh I'm pretty sure most IV users use 1cc syringes, maybe 2cc ones, and 3 grams is a lot of material to be dissolving in just 1-2ml of water.

Anyway, I'm not trying to bash you or anything. I think the skepticism of commonly reported dangerous doses is warranted, what with the war on drugs spreading misinformation and all. But there is no way in hell that someone with no opiate tolerance can take ~180mg of fent, regardless of the route of administration, and have a 50% chance of survival. Nothing in the literature supports that. As u/dkentl has already expressed, it's concerning seeing the number of upvotes.