The four, including an account director, left the offices last week. They equate to three per cent of MS&L's total staff, which numbers about 130.
MS&L lost the account four months ago, although the Omnicom consortium that won the business did not start work on it until this month.
MS&L had a team of 20 working on the account, believed to be worth £5m-£7m globally. It previously held 85 per cent of that business.
Kate Steele, director, new business & marketing, said the remaining 16 members of the Philips team had been redeployed across the business.