Jeremy Fraser - leader of Southwark Council from 1993 to 1997 - and Jim Dickson, who led Lambeth Council from 1998 until 2000, are to join Four early next month.
Four managing partner Nan Williams said the agency saw growth potential in public affairs and further hires were planned this year for Four's PA team.
The agency currently has just one dedicated public affairs consultant - Mike Pooley, who worked with Fraser and Dickson at Weber Shandwick, from which he joined Four six months ago - among its 21 staff.
The agency's largest public affairs client is airline bmi, which is an account that Fraser and Dickson will work on, Williams said.
WS has promoted associate director Tony Hutt and account director Luke Akehurst within the London PA team. Joint MD Jon McLeod said WS was also on the hunt for a 'senior hire on the Lib Dem side'.
Fraser - who was campaign manager for Frank Dobson's failed 2000 London mayoral bid - said: 'I've learned an enormous amount at Weber Shandwick and at GJW before that. But I felt it was time to move on and run my own show.'
Fraser stood as Labour candidate for the North Southwark and Bermondsey constituency in the 1997 general election, losing by just over 3,000 votes to the Lib Dems' frontbencher Simon Hughes.
Dickson - who joined WS in May 2000 - stood as Labour candidate in former Prime Minister Ted Heath's Old Bexley and Sidcup constituency in 2001, losing by 3,345 votes to new Conservative candidate Derek Conway.
Both Fraser and Dickson join Four as shareholders in the agency and retain their WS job titles of director and associate director, respectively.