We have four DC, two of whom are disabled (one diagnosed before 4th DC and one diagnosed since). Our fourth DC was unplanned and I did consider ending the pregnancy as I had done with a pregnancy when DC3 was a baby but I decided not to.
In terms of the day to day parenting and meeting their needs, nothing phases me (or DH) because we've got the experience if having been through each stage several times already. We know the rough patches pass and it's just a case of powering through. We know what our red lines are, what are expectations are, and what works for us as a family.
Practicality-wise, it's expensive. All four needed new shoes for school and because their feet had grown so last week I bought school shoes, playing out shoes, and wellies for all four DC plus PE trainers for the oldest three - one hour later and I was £390 lighter. We go through near on a loaf of bread and four pints of milk a day. We're having a chippy tea tonight and it'll cost in the region of £30+. We need a three bed apartment or two hotel rooms when we go on holiday. I drive a seven seater car. People make judgements about how they must all be out of control, that they don't get enough attention, that we must not have a TV, that we must be religious, and so on.
We're not having any more, especially as there is a genetic element to my DS's disabilities and our chances of having another child with the same condition(s) are higher. Even if we didn't have that to contend with, four feels like our limit. It's manageable and enjoyable, I think five would tip us over into being stressed and tired which wouldn't be good for any of us.