Pippa Glucklich, UK chief executive of Starcom, Nikki Mendonça, EMEA president of OMD, Tracy de Groose, UK & Ireland chief executive of Dentsu Aegis Network, and Paul Frampton, UK & Ireland chief executive of Havas Media Group, are all going.
Mendonça is joining Accenture in the US but the others have not announced new roles.
The scale of the agency exodus has been dubbed "Shocktober" by Bruce Daisley, Twitter’s European boss. There will be more changes.
There are unique and personal circumstances behind every departure but it is possible to discern a wider theme.
Agency life is hard and getting harder in the big ad groups. And a growing number of senior people have had enough.
"It’s brutal – there’s no buffer," one agency chief, who is not departing, says.
There are multiple challenges: pressure to deliver financial results is relentless and the UK ad market is going sideways after Brexit.
Corporate politics can be ugly in a multi-national, multi-agency holding company.
Agency groups have become too big through acquisition and need simplification and fewer silos and layers of management.
The transparency debate and intense scrutiny from brands about how agencies have been making money in undisclosed ways have also hit margins and morale.
One agency leader describes feeling hurt and upset about being labelled as untrustworthy and untransparent.
And yet this person works for one of the big six ad groups – all of which still refuse publicly to back ISBA’s tougher and more transparent media agency framework contract, 18 months after the trade body for UK advertisers first launched it.
If the ISBA contract is good enough for the government’s £140m media-buying review, which has put transparency "at its heart", it should be good enough for agency groups.
It is significant that two independent agencies, Goodstuff Communications, the winner of Agency of the Year at the 2017 Media Week Awards last night, and the7stars were among those invited to join the giant agency groups for a joint face-to-face meeting with the Cabinet Office yesterday about its media review.
I have written recently that this is the start of a new era for the big media agencies.
They have got to change. And yet most of the leaders at the very top aren’t articulating a convincing argument about how they can change, even as they watch consulting and tech firms enter their world.
No wonder some agency people are ready to quit. One headhunter says they have seen an increase in phone calls and inquiries just in the last week.
It’s not just ambitious folk interested in new job openings. A few are wondering if they need to get out of the big network agencies altogether.
There is too much negativity around the agency model, and it’s going unchallenged by agencies, which employ thousands of dedicated, talented people.
I tried to make this point when I spoke at the Media Week Awards: "If you believe in the importance of media, if you believe in the power of building brands, if you believe in the value of trusted, quality content, then the media industry needs to stand up for itself.
"The media industry needs to be proud, to be more open, to be more transparent.
"I look around this room and wonder which companies will be here in five years' time? And I know the answer: Only the companies that change."
To change requires leadership.
Gideon Spanier is head of media at Campaign