There is no way for the parent to bring up 4 chicks and even at 3, it is already very difficult. I haven't found one nest having 3 eggs in my BHs and maybe because I never take unused nests and always allow the birds to fledged before taking the nests....unless your is the hot dog type of nest which is made by afew pairs of birds at the same location. Maybe, your's is the starting of a new species of EBN swiftlets and since you are the founder;  you got the right to give it a name.........hahahaha.

Although there are really many possibilities, maybe some here have similar happenings and can give an reason for the 4 eggs in the " nest riddle".

Taking about EBN price, here, it was Rm4700, 5 days ago and Rm4600 3 days ago but some information that other places are selling them @ Rm4300, wonder if it is the work of Indo magician @ work. I should have sold mine for Rm4750 when offered last week....sob