We were co-sleeping with our toddler, when baby number 2 arrived (not literally, mind!) and still are now baby is 9 months old. Toddler always goes to sleep in his own room but comes in with us if we have gone to bed when he wakes. (If we haven't gone to bed, we resettle him in his bed). Like you, we had tried (gentle) sleep training with him, but he always wakes at least three times in the night. He sleeps very well in with us, although he still stirs we can settle him easily.
When baby number 2 arrived, she was in a crib by our bed. It just didin't seem right that this tiny baby was seperated from us by a crib, and our toddler was cuddled up in between us, so I purchased a bedside cot for baby. I was/am breastfeeding baby, and although she starts the night in the bedside cot, she always ends up side by side with me, so we do have four in a bed!
Our toddler still doesn't sleep through, but he is sleeping for longer and longer in his own bed and we are hoping he will eventually sleep through, but until he is ready to (and we believe this is a developmental thing, rather than a sleep training issue), we are happy to bring him into bed with us.
On the whole, we are happy with this sleeping arrangement and it means we all get enough sleep.