I don't think they're necessarily rebelling against you because of them feeling like they've been taken from your dad, I suspect they're showing the effects of trauma from having seen emotional and physical abuse, heard it from their rooms, seen the impact it had on you, etc.
This is likely to have made them stressed and scared and made then develop their own responses to cope. As they are close in age I expect they find solace in solidarity so do a lot of naughty behavior together. They have also learnt inappropriate norms about how conflict is resolved.
Even though they are now safe and you are protecting them from further harm, it all take longer for them to be able to have full confidence in that. I imagine their behaviour comes out of being constantly alert, something people call hypervigilance. This is born out of fear and a feeling that something might happen just round the corner. Children who have experienced trauma often appear very similarly with ADHD and some research suggests that lots of children may actually be misdiagnosed as such.
Unfortunately there's no magic solution as you know. Take the advice given here, make sure your boundaries and consequences are as crystal clear as possible. But also make it age appropriate- a 3yo is unlikely to understand being sent to their room for a long time like 20 mins or 30 mins.
Try deescalating before things get too bad by distracting with a different activity, even setting if the children are able to recognise that they're at risk of doing something naughty and have a fun time out space with maybe drawing and things they can squeeze and feel.
Try and make sure the messages they get from you are consistent, and that ultimately they know you will keep them safe. They might start to relax.
And for that final reason I agree with pps about going away to Canada. It's easy for an older child or an adult to process a loved one going away for a week. But far harder for a 3 and 4 yo, and you are the most important person in their lives by far. I think any child of that age, but especially in your case because of what I've said above, they need absolute consistency. That's really hard I know.
Well done for getting them, and yourself, or if the situation you were in.