MD Lucy Hackett, senior account director Kate Shanahan and account directors David Pugh and Alexandra Nelke will all leave the agency by September. None of the four has a job to go to.
Sources close to the agency claimed the departing staff were setting up a new agency called Marmalade.
Hackett denied this, saying: 'I don't have any plans to set up an agency at the moment.'
But she did not rule out working with former colleagues in the future. 'I'd never rule it out,' she said.
Founding partner Kate Waddington said she would not replace the three account director positions on a like-for-like basis.
Brignell joins the agency at the beginning of September as MD. She has previously been associate director of Henry's House and CEO of Revolver.
Sputnik recently handled media for the relaunch of Cadbury's Dairy Milk brand, promoting the now famous advert featuring a drumming gorilla.