She sounds normal, which doesn't mean it isn't tough! Someone once said to me that every child has an age where you have to really put the leg work in - some are dreadful tots but relatively mellow teens. I'm hoping so because my 3yr old is wild!
Seconding "How to Talk..." and also really like the website aha parenting. I'm really trying t move beyond shouty mummy and a technique that really works for us is separating the child from the problem, so basically when something happens that would usually kick start a row, instead of saying, "You've spilt the juice!" you say, "we've got juice all over the floor, pass a towel!" I find it especially useful at keeping me calm and solution focused. It works in all sorts of situations, I even use it to get him ready to go when he's fussing "we've got a problem, x starts in half an hour and we aren't ready to leave!" For us, it works because when we get into a negative interaction, where I'm telling him off or nagging, it usually escalates into a row and tantrum.
Perhaps you could start to look specifically at behaviours or interactions you want to change, and work through each. MN is great for actual strategies for individual behaviours or problems, books can be useful and you could ask nursery what is going well for them.
One thing though OP, you have to drop the thought that she has the power to wreck your relationships. Only you and your DP have the power to break up or stay together, if he couldn't handle her, or you two could not work out how to relate to her and each other peacefully then that's your and your DP' s business, not hers. She's just being a four year old, she's behaving how tiny, irrational people do, she is not the keeper of your relationships, and you not she has the power to mend or break them. Do not make her a scapegoat, it is unfair to her and will also not in any way help you either mend your relationship to her or give you the ability to maintain your other relationships.