CoolyFooly
Hi mate welcome to the forum
I'm inclined to agree with Babs here, societies will all pile into one small nest, it's comical watching em

lols,
However if one has layed an egg in the nest sooner or later the egg will probably get broke,
The bird that is now sitting out on the perch could possibly be the hen that actually layed the egg, she probably won't feel comfortable in sitting in the nest now because she possibly knows that it's now to cramped,
Two birds in a std size nest with egg / eggs is fine but with 4 birds it now changes everything,
The bird sitting on the perch obviously may not be the bird that layed the egg as we'll never really know for sure but I wouldn't worry to much,
What you could do is actually leave the egg in the nest for a few days and then take the egg and carefully hold it upto the light and if it looks full then place it back in the nest,
If the egg looks clear ( empty ) then simply disguard it
Personally I'd say it's most probably clear but give it the benefit of the doubt at least for a few days