I have three, although my youngest is only four months old so I'm sure there will be rocky times ahead - but, I absolutely love it.
I am so much more confident and relaxed with this baby in terms of things like sleep routines, breastfeeding, bedtimes etc. My older two love their baby brother so much and haven't seen any regressions in behaviour or anything like that this time around.
Bringing my third home to meet his brothers and my mum was, no exaggeration, the best moment of my life so far. The whole room was full of joy, I can't explain it but I just knew my family was complete at last, and everyone was there and healthy and so happy. It was magic.
I have similar gaps to you - when my baby was born my eldest was 2 weeks off his sixth birthday and my middle was about 3 and a half. So the bigger two are in school/preschool while I'm at home with the baby. By the time my maternity leave ends, they'll both be in school so I'll only have to pay for one lot of chilcare.
My older two are pretty good at playing together fairly independently, and they're both now reliably toilet trained day and night, and sleep through most nights. I'm finding it a lot easier than the last time I had a 4 month old, when I was trying to toilet train an unruly 2 year old at the same time while dealing with a sleep regression in a heatwave!
I had a massive panic when I found out I was pregnant with him and it wasn't the straightforward, happy news I'd had when I found out with my older two. In fact we seriously considered ending the pregnancy. I'm really glad I didn't, although I am 100% done now and DH is getting a vasectomy, which I'm happy to support.