Breast is best, but a happy & healthy mum is even better!
You've done amazingly well to bf not just 1, but 2 babies for pretty much the past 3.5 years- I think you should be really proud of yourself as that's quite an achievement!
However, at the age of 3.5, your DS no longer needs the nourishment of night-time feeds- it seems he is waking for comfort feeding/ being close to you during the night?
If you want to stop night- feeds, then there is absolutely nothing wrong with that, and you shouldn't ever feel as though you're failing by not having him sleeping through the night, as it sounds as though you're a lovely Mum who's put her babies comfort & contentment first for a very long time.
At his age, can you try explaining to him that from now on you'll have a nice bedtime story & last bf of the day (in his room or yours- whichever fits better), and then no more bf til morning- you could try a sticker reward chart for sleeping in his bed all night/ or stickers for gradually decreasing the amount of times he comes in to your room for a bf/ loads of praise in the morning for staying in his bed. Perhaps a new soft comforter toy that he can have in his bed to hold & stroke while you do story/ last bf, so he has something else to associate the comfort feelings with during the rest of the night in his own bed?
Feel for you- it must be hard to help him break the habit, and it may take time to get him consistently sleeping through. But it's far more important for you as a mum of 4 to be rested/ happy/ unstressed- you've done brilliantly so don't ever feel down on yourself! 
