Foreign country + foreign language + nighttime + a four hour cab ride + no prior knowledge of the route = YANBU
I lived many years in a country that didn't view women highly - or to be more precise, many men didn't view women highly at all, but some were more normal, meaning strange men could be a dangerous lucky dip. I spoke the language and knew my way around and I would never have done 4 hours in a taxi with a driver that wasn't known to the organisers or very well to me personally.
Maybe your friend has been there before and done a lot of solo travel in that country though?
Like others said it's not necessarily about sexual assault, which is a risk, it's about ending up in the middle of nowhere, and being asked for money. Or arriving at a cousin's house for a meal along the way that takes AGES...that you then need to pay a lot for. Or arriving at a shop that you need to buy things you don't want from or he won't carry on the journey. There don't need to be any weapons or direct threats. You're completely in his hands. And of course, when you're in these situations, you don't know what's going to happen next. Once you're in the car, you don't know what's going to happen until you arrive at your destination. If the car stops en route, you are completely at his (it's never a female driver) mercy.
It's a very vulnerable situation to be in.
And I'm betting that in this country two women of your age would not take a 4hr taxi ride through remote areas in the middle of the night. And their friends and relatives wouldn't suggest they did.
So it's a dangerous lucky dip. You may very well be totally fine. But if things go wrong you're not going to be able to respond in the same way you would in your own country because you won't know where you are, who to trust, or how to speak the language.
And anyway, do taxi drivers want to drive 4 hours away?!
The important-event organisers or venue must have drivers they know of, or contacts who do. Pay more and use one of them. It's not a cast iron guarantee, but you take yourself out of the dangerous lucky dip then.