Past 4 days late and you have a greater chance of having chicks born with lots of problems and a high number that won't live or need to be culled. It's very rare to have chicks born past day 24/25 without problems. My first hatch I trusted my thermometer (never checked it for accuracy). It read 99.5-100.5 for the whole incubation so I thought all were good, they were developing and moving. When I went into lockdown I thought they looked a little behind but I was hoping it was just my inexperience. I finally had one hatch on day 24 and one hatch on day 25. The day 25 chick died before 24 hours was up. My day 24 chick was spoiled. I thought he was healthy, but as he grew I realized he wasn't right. His balance was off and he wouldn't/couldn't roost. Upon closer inspection I noticed his "knee" joint was turned in. Most likely because of the delayed development. Before I set again I checked the thermometer I used and found that it was 6 degrees off. So, when I thought my brand new thermometer was reading right and my temps were 99-100F, they were actually only 93-94F for the entire time. This was the cause of the delayed development and late hatch. He was 3 days late and showed problems-I'd hate to see what a chick 5-6 days late would be like. If you incubate in an incubator-never trust any thermometer that hasn't been checked-even brand new.
My advice whether it's an incubator or broody- by the night of the 23rd day, pull them out and candle. If there are no internal pips and no obvious life/movement inside the egg consider doing eggtopsies or tossing. I could never throw out an egg that still showed signs of life, I'd have to take my chances, but if there is no sign of life there is no point dragging out the inevitable.
If the eggs are at (internally) a high temp for an extended amount of time, it can cause death to the embryo.
As for breaking a broody, I haven't had to deal with that. There are some good broody threads on here though that might help with that.
