World Health Organization 2019:

Substance identification:
Para-methoxy-butyryl fentanyl is a synthetic analogue of the opioid analgesic fentanyl. Samples obtained from seizures and from other collections suggest that para-methoxy-butyryl fentanyl occurs in powder, tablet, and nasal spray forms.

Effects on Central Nervous System:
Reported clinical features of intoxication in which para-methoxy-butyryl fentanylis involved included the typical opioid effects of reduced level of consciousness, respiratory depression and pupil constriction. In some cases, treatment with the opioid antagonist naloxone was shown to reverse the drug-induced respiratory depression.While this is consistent with an opioid mechanism of action, it should be noted that in all such cases at least one other opioid was present.

Recommendation:
The limited available information indicates that para-methoxy-butyryl fentanyl is an opioid drug, and an analogue of the opioid analgesic fentanyl. There is evidence of its use in a limited number of countries with few reports of intoxication and no reports of deaths. In the intoxication cases, the role of para-methoxy-butyryl fentanyl was not clear due to the presence of other opioids. It has no therapeutic usefulness. At this time, there is little evidence of the impact of para-methoxy-butyryl fentanyl in causing substantial harm that would warrant its placement under international control.

(PDF page 3 / 22 pages) (41st Expert Committee on Drug Dependence):
https://www.who.int/medicines/access/controlled-substances/Annex_1_41_ECDD_recommendations_NPS_and_medicines_22012019.pdf

Reddit Search Results 4-MeO-BF (7 Results):
https://www.reddit.com/search/?q=4-MeO-BF

Novel opioid 4-Methoxybutyrfentanyl (4-MeO-BF) (2015 Thread):
https://www.bluelight.org/xf/threads/novel-opioid-4-methoxybutyrfentanyl-4-meo-bf.755971/

4-Methoxybutyrylfentanyl Monograph (PDF 5 pages):
https://www.swgdrug.org/Monographs/4-Methoxybutyrylfentanyl.pdf

WARNING This product is not for human or veterinary use.

para-methoxy-Butyryl fentanyl (hydrochloride) Safety Data Sheet (PDF 5 pages):
https://www.caymanchem.com/msdss/18089m.pdf

An opioid analgesic that is an analog of butyrfentanyl and has been sold online as a designer drug. Side effects of fentanyl analogs are similar to those of fentanyl itself. Life-threatening adverse reactions have been observed.