You're not doing anything wrong. Kids are different and have different patterns. It's possible that, because he was with you longer, he got into this habit, but it's also possible that he's just different.
You're also more exhausted because you have 2 others this time around.
I never used a paci because they can't self-soothe with it - they can't get it into their mouths.
I did sleep training on my pedi's recommendation. She said they make up for food intake during the day. I know not everyone likes cry-it-out and there are valid arguments on both sides. One argument is, never let a child cry, just feed him. Another is, kids need to sleep and so do parents - we taught self-soothing with back pats and soothing words but no picking up. I also had a baby monitor so I heard him sooner in the process. Some people find it helpful to alternate mom visits and dad visits, and more with the dad visits if you are nursing since the baby doesn't see dad as a source of breast milk!
Is it possible to move the 4 year old in with the other child for a few days while you try sleep training? We had 3 nights of hell and then everything was fine.