Not smarter ;-), by far, it is a guess to be tested, based on older kadmelia networks like guntilla/emule where 8/20 replicas was enough, but when all connections were very light, i…e not checked for many hours/weeks/months between churn events. As we are milliseconds between churn events then the chance of 4 nodes going down in the average churn event seems unrealistic. This is good, but potentially too good, we may not need 4 copies (kademlia republish is 24 hours, refresh == 60 mins). 4 copies may be way too much IMHO.
The bottom line for us. is that we lose no data, beyond that is just more caching really and not necessary.