What do you do if your young child refuses to brush their teeth?

I am at a loss with what to do in regards to brushing my almost 4 year olds teeth. I have always brushed her teeth and she has always hated it. My husband and I have tried so many things, we have tried changing brushes (electrical, manual, different bristles, etc.), we’ve tried playing games like having to get shapes/unicorns/bugs out, we’ve tried letting her pick between us as to who will help her brush and no matter what we do she just clamps her mouth shut or will yell and cause a problem. We can wait for up to an hour, trying to reason with her and trying different things and she will not let us brush. We have gotten to the point that after we try a few things if she still isn’t letting us we just give her one last warning and say if you don’t open up we will have to gently hold your arms so we can do it, oftentimes she still won’t which results in one of us trying to gently hold her and brush while she screams and makes a scene (even though we know we are being gentle). It makes me feel horrible and I hate that it is such a negative experience but I dont know what else to do. Am I supposed to just let her get away with not brushing and have her teeth rot? Any tips would be greatly appreciated.