The 4th covid wave is already in the US. It is fueled by the much greater infectivity (1) of the Delta variant arriving at a time of nationwide reduction in masking and social distancing in part driven by what a increasing number of public health officials believe to be a premature and widely misunderstood lifting of the mask mandate\by the CDC.
Some virologists are estimating the R naught of Delta variant to be as high as five. Anecdotal report from Australia indicates that viral transmission occurred with simply passing an infected person in a mall.That seemed to me unlikely until I read of the Chinese study demonstrating a viral load in Delta infected persons greater than one thousand times that of the original Wuhan strain. Their report found that high viral loads persisted up to 4 days in asymptomatic patients. Asymptomatic spread from persons with high viral load is pretty good if not perfect storm for spread in a population who just recently were officially relieved of need to mask or distance.
The report of transmission of the Delta variant among fully vaccinated people at an outdoor wedding event in Houston seems to strongly indicate that transmission can happen in fully vaccinated people which should mean that not just the unvaccinated should wear masks in public but so should the vaccinated.
The vaccinated should wear masks in public for two reasons; 1) they can spread the disease to the unvaccinated which includes children who are too young to be currently eligible for vaccine and of course the elderly and immunocompromised 2)there are "break through" cases i.e cases in the vaccinated These are usually described as "mild" but some are not and there have been deaths largely in the elderly.
Israel quickly reacted to increased Delta cases by reinstituting a mask mandate as did Los Angles County in California. The medium term answer is more vaccination but the short term answer is masking and social distancing .Already governors in Colorado,Florida and Texas have said they will not mandate masking for the public and will not "allow" mask mandates for kids returning to school although that is the recommendation of the American Academy of Pediatrics.
With this wave it looks like there will be fewer deaths in the elderly ( mainly because of the higher vaccination rate in the old folks) but reports from Alabama, Arkansas and Louisiana hospitals indicate that ICU admissions and some deaths are occurring in the younger group. Whether that indicates a greater lethality from Delta is not determined at this time.