I've never heard of this. I didn't have a 4-hour curfew when my son was 10.
So your daughter cannot go to the beach/lake for a day, cannot work a job, never went to a slumber party at a friend's house, can't go to school/college? What is magic about 4 hours? If she's going to get into trouble, she can easily do it in 2 or 3.5 hours.
If she ran away and has come back, then she understands the value of family. I can see a REASONABLE late night curfew especially if her coming in very late will wake the whole household (for example, if you have a dog who goes nuts barking when the garage door goes up or something like that). I think it's common courtesy and decency to let people in the household know where you are (more or less) and roughly what time you'll be back so that there's a way to know what time one should worry or whether one should prep dinner.
But other than trying to control her, stop and ask yourself how a rule like this is preparing her to be an adult and manage her own life? She's 18, nearly 18.5, and you want to restrict her in this way? How will she ever learn to be responsible enough to be on her own, out in the world? What is your plan to teach her the skills that, somehow, you have neglected to teach her at 12, 14, 16, and now 18? Honestly, if she cannot be out and about like this, it is much more of a reflection on you and your husband failing as parents to teach reasonable life skills.
Is this some sort of payback or revenge for her running away? And if so, do you think it's more about you? Do you know why she ran away? Do you think she was rebelling against unreasonable restrictions but didn't have the life skills to make it on her own? So she came back? Stop with the rules and start with the adulthood. She pays her phone bill, has a job, does chores, and purchases household supplies, and you treat her like a 6 year old? Please get some family counseling to help with this. Do you have other children who are growing up the same way? This is a disaster in the making, turning out new adults who cannot survive because their parents wouldn't let them live.