It is possible that all 4 are laying, but don't expect all 4 to lay every day. maybe 1 or 2 take a break from laying, and resume the next day. Another thing to consider is that chickens lay on a 25 hour cycle (most).. That means that once it gets dark, and she did not lay an egg, expect it the next morning. This rotation can throw of the egg/day pattern.
There is another method to see which chicken laid which egg. It involves putting some food dye onto a chickens vent. (not way inside) The egg will have a streak of the food coloring on it. You would need to use 4 different colors, one different on each hen. Observe for a few days to see which hens are laying. The food coloring is supposed to last a few days. I newer did it but read others that tried it with success.
AND OF COURSE