Honestly it's a stage. My girl went through it around 3.5, smack in the middle of covid so who knows whether that played a part but it certainly made being around her all day every day doubly challenging! Like you I had always enjoyed an incredible bond with her and couldn't understand why she was suddenly so bloody furious with me all the time and seemed to go out of her way to wind me up. It's so upsetting.
But it's upsetting for them too, though they may not show it. They're growing up, they're confused, life is full of change. Can you remember being 4? I have quite good memories of the age and I remember being so emotional. Fear felt like I might die, anger felt like I might explode, being upset - oh God, I'd cry so hard it felt like I couldn't and might never stop. The only stage equivalent I can remember is my early teens.
In terms of just Getting Shit Done (when even the most simple requests are met with 'no!' and faffing around) the best book is 'How To Talk So Little Kids WIll Listen'. The tips in that for getting them onside (making them laugh in the middle of their strop is a really good one), and for keeping your temper when it just doesn't work, the 'scripts' you can use instead of letting your anger and frustration out (which never actually yields results, escalates the situation and makes you feel like utter crap afterwards) are so so helpful. Whenever I've had a shitty day with my DD (who is 5 now and much better but OH MY we still have our moments) I'll think to myself "Damn I really need to read that book again and start using the tools."
Right now you're ill and tired and so is she. Forgive all of yourselves and be gentle to yourself. You may also be like me and very reactive to other people's feelings. So it is utterly horrible and feels like the end of the world. But honestly, she is still your lovely girl. She's just going through something, and doesn't yet have the maturity to consider how that is impacting on others around her. She doesn't have the tools or the insight. You are the grown up and can find tools and ways to help you all survive this with your sanity intact.
One thing I will say is don't take responsibility for the impact on your husband. He is her parent too, he doesn't get to 'work all day' then expect to come home to peace and harmony - he has a child, she's going through something, it will be hard and it is NOT YOUR JOB to whip her into line for his convenience. Get him to read the book too and present a united front. Learn to tolerate his upset, and hers - you have your own upset to manage, you can't be expected to be the emotional sponge for the whole family. But if you can learn to lead by example, to control your reaction to her behaviour and show her and teach her how to find her way out of it, then she will eventually pick up on these skills.
Hang in there. It gets better. She is still in there.