Everything is a phase when raising children, and everything passes eventually. The trick is to ease a problem behavior and not replace it with something even worse. It's hard to tell from the information you've given, D., but to me this sounds like insecurity, and could indeed have been precipitated by the events you mention, and possibly made worse by your daughter becoming overtired or overstimulated during the relatives' visit, or just during the day.
If this is complicated by some irritation in her body or surroundings (too cold or warm, stuffy nose, tummy problems) your 4-year-old could be too uncomfortable, emotionally and/or physically, to sleep. Reaction to foods or food additives have also been shown to be upsetting to children: common food colorings and preservatives have recently been proved to stimulate sensitive children.
It's understandable that you feel desperate, but it sounds like your daughter is, too. Please remember that she is only four, and doesn't have the experience or impulse control to know how to deal with her circumstances. She is expressing a need as clearly as she is able. Comforting a child in distress, even if this is needed over days or weeks, does not "create a monster." Taking away privileges or favored toys will not deal with anxiety, if that is the trouble, and could confuse her or make her feel even less secure.
At four, she might be able to talk to you about what's going on at night. I'd try this conversation during the day first, asking if she can remember what woke her (A dream? A full bladder? Some discomfort?). Without putting words in her mouth or judging/criticizing what she offers, ask her if she can tell you what she thinks about during the night. That could suggest a solution.
If you have fallen into a "convenient" routine during the day, maybe lots of television and not much healthy exercise, try working on that. She might need a new bedtime routine, maybe starting earlier in the evening to help her unwind and feel loved and cozy. Stories or songs about bedtime and happy families/kids might help. She absolutely needs and depends on your care and cuddles, and probably will until her teens.
I'd like to observe that one comment you made raised a possible flag for me: "My partner and I can't take it." Please disregard if this doesn't apply, but did your partner come into your life after your child? If he/she is running any sort of competition with your daughter, for your time, attention, delight, or care, your daughter is getting old enough to become uncomfortably aware of this. Such a competition might exist in reality or only in your child's mind, but at any level, it needs to be addressed if it is there. Your child desperately needs to be your highest priority until she learns to be more independent. If you suspect that might possibly apply, please find ways to let her know you are her mommy first. Ignore this, and you are looking for years of trouble.