are you sure the hives didnt swarm and the new queen just hasnt started laying yet. Another option may be that in the hot weather the queen will sometimes shut down. if there is an extended period of hot and there is a dearth on
The chances are that if all 4 hives went broodless at the same time it is because of a nectar dearth. Both Russians and Carniolians will into a brood dearth in the advent of a nectar dearth, Russians will even do it during a slow down of the flow. They will also explode seeming instantly when a nectar flow starts.
The othe possiblility is that all 4 hives swarmed with in a short time period since the last inspection.
Swarms are evident from lack of eggs/lavae, or even capped brood, the bees that hatch since the swarm replaced the bees that went with the swarm so the population of live bees can never be used as a guage of swarming. The hive will continue to act queenright, calm, quiet, and industrious.
Also, I'm seeing evidence, especiall with those races of bees mentioned, that they purposely induce a brood dearth of several weeks in mid summer to control varroa.