I will be sober 18 years in 3 weeks (God willing). If your STBXWH is serious about getting sober, he will be going to AA, start working the steps, and get a sponsor. Not drinking is not getting sober, it's merely white-knuckling it until he feels safe enough to start drinking again. He's still in denial that he is an alcoholic.
This may seem counter-intuitive, but the best thing you can do for him, alcoholism recovery-wise, is to let him hit rock bottom. And let that bottom be deep and hard. Just like infidelity, there have to be real and painful consequences to his actions before he'll change.
It's hard as hell to get sober, and it takes a LOT of motivation to put yourself through the hard work it takes to get good sobriety. And then you have to diligently maintain that sobriety.
So yeah, don't feel guilty about leaving him without a support system. He needs to experience that loss. You said it yourself that it was the drinking and his behavior while under the influence that was the straw that finally broke the camel's back.
I'm not saying you should be callous or uncaring. I'm saying don't be co-dependent or try to diminimish or lessen the consequences of his drinking. Just like the affair, you didn't cause his alcoholism, you can't fix it, and it's definitely not your job to fix it. It is 110% on him to get his shit together.
An Al-Anon meeting might be good for you.
Keep on your path and take care of yourself and your kids (especially your 4 YO) first. Your STBXWH should be a distant third. (Actually, he shouldn't even be in your top 10.)
The good news is (even though it's pretty sad good news), less than 5% of marriages survive one or both spouses getting sober, so at least you're not trying to swim upstream on that one.
You're doing really well is a super crappy situation. Keep up the good work!