DD was a champ sleeper. One night I did CIO to help her learn to self soothe and that's all it took. She did 3 hour naps till she was 5 and still sleeps 12 hours at night, including going to bed by 7pm. One of the boys is a pretty good sleeper, minimum wake ups, goes to bed around 7, up for a bottle at 5, back to sleep until around 8. Naps at least 2 hours in the afternoon.
The other? Usually wakes up a couple times a night and needs a nippie back, sometimes has to cry for a bit around 4 am. Bottle around 5 then up for the day at 6:30-7. I could even handle that schedule if he napped. but...
Naptime is a miserable experience. I put him in his crib and some days he goes down easy-but only for about 30-45 minutes. Then he'll scream until I get him out. He is EXHAUSTED! Eye rubbing, fussy, etc. If I sit with him he'll fall asleep in my arms, but in his crib he will scream for hours if I let him. I've tried going back in every 10-15 minutes, reassuring, patting, etc. I've tried leaving him be on my most frustrated days. Tylenol has been given, temps checked-he should be fine. Why won't this kid sleep? What do you do when CIO fails? I can't hold him to sleep for his whole nap every day.
Re: did anyone have CIO not work?
My oldest has always been a horrible sleeper and had night terrors when he was younger. We did CIO with him and it would work for awhile until we had some sort of transition (illness, vacation, etc) and then we would have to start all over. Even when he "sleeps through the night" now he still wakes up once or twice just because he has nightmares or whatever. He will be 4 in a couple of weeks and this is the best he has slept in his life.
It worked a lot better with DS2, but he was a better sleeper anyway. I really think some kids just have a hard time sleeping and there's not much you can do but let them grow out of it.
ETA: naps. Ugh. DS1 stopped napping just after he turned two. I have no advice, sadly, because I really think some kids just fight sleep or have a hard time with it. Is it possible he's just teething or working on walking or some other sort of phase/sleep regression that he'll get out of in a couple of weeks?
Some kids release tension by crying, others increase tension by crying. Sounds like that might be your boy? Ask Moxie talks about it here - https://www.askmoxie.org/2011/01/tension-increasers.html
My girls did well with CIO when they were 6 months old, but when I tried to do it again later around a year it was a complete bomb, and only worked both of them up. I don't have a lot of advice for you other than that age 4 is definitely when nighttime sleep takes a turn for the better. I do think if you can recognize that CIO won't work, it may open you up for other options to soothing him.