I’m so sorry for your loss.
Unfortunately bunnies are prone to stomach problems, and often there’s nothing you can do. I don’t know if it will make you feel better, but a few posts above yours someone lost their bunny in a similar circumstance, despite brushing everyday. As bunnies are such obsessive self-cleaners it’s really really hard to completely stop them from ingesting fur.
She would have been anaesthetised when she died, so she wouldn’t have known you weren’t there. You were doing your absolute best to save her. I’m sure she had a very happy 4 years with you, and it sounds like you did everything right. Try not to beat yourself up, you obviously loved her very much and you did everything that you could.
My beloved bunny Sirius had an overnight stay at the vet after a gut blockage. They called me at 6am and said they had to put him to sleep immediately because he was suffering, and there was no time for me to get there. I beat myself up over it for a long time, and it still makes me cry, but you have to focus on the happy life that they had when they were with you.