"I've had a bad family upbringing and unstable relationships"
This, to me, says it all. He's selected you as his next victim as he sees you as supplicant, pliable, mouldable. He'll treat you like shit and move on. Or be locked up again.
As PP have said, if he's doing 4 years, that's no first offence, he's lying to you. And / or he's done something bloody horrendous to his ex.
If he's been banged up for 4 years and they only split a year ago, that tells me that she was still under his control and only managed to get away 3 years after he was imprisoned.
He's been sober for 4 years.. OP, you cannot be that deluded to think that this was voluntary - he's been sober because he hasn't been able to get alcohol in prison. One thing that I can tell you about alcoholics: forced sobriety does not last.
Do yourself and your kids a massive favour - block and delete. He'll bring nothing but misery and pain to your lives.
I hope that you take serious note of some of the advice given to you here so that you and your DC don't end up another statistic.