When a cat begins to eliminate outside of the litter box, it is its way of telling its owner that something is wrong.
You need to determine whether the problem is physical, or behavioral, or both. This means that you must bring Amber to the vet as soon as possible, so that the vet can pinpoint any physical issues that Amber is having. The most common physical reason for urinating outside of the litter box is a urinary tract infection, which is treated with medication. Basically, the cat experiences pain when she urinates, and associates the pain with the litter box rather than the infection itself, and as a result she seeks to avoid using the "painful" litter box. By any chance have you detected blood in the urine or stool? That would be clear evidence of a UTI.
There are many possible behavioral reasons for Ambers behavior, too. She might be stressed. You have four cats; how many litter boxes do you have? The general rule is one box per cat, plus one extra. So, ideally you would have five boxes. Have you changed the litter lately? Have you changed the location of any of the boxes? What are you feeding Amber? Has that changed lately? Are you using Feliway diffusers or the spray, or Rescue Remedy to help calm her?
If you provide forum members with as much detail as possible (including answering these questions), we might be able to zero in on the likely cause(s). And I would imagine that other members who are more knowledgeable than I am about this issue will be along soon with their thoughts. I should also point out that there is an excellent sticky entitled "Inappropriate Peeing Problems Answered" in this forum (third from the top of the list). But I want to emphasize that none of this is a substitute for getting Amber to the vet ASAP. If it's a physical issue it is not going to clear up on its own and UTIs can be quite serious if left untreated for too long.