Prime Minister Stephen Harper has added a fourth byelection for September, as speculation grows that the Conservatives are gearing up for a general election later this year.

Voters in the Toronto riding of Don Valley West will elect a new MP on Sept. 22. The seat was previously held by Liberal John Godfrey, who resigned on Aug. 1.

Harper has already called three byelections for Sept. 8, including the Guelph riding of Southwestern Ontario, and the vacant Quebec ridings of Westmount-Ville Marie in Montreal and nearby Saint Lambert.

The Conservatives have legislated a fixed election date of October 2009. But Harper and Liberal Leader Stephane Dion have both suggested they want a general election much sooner, with Harper going so far as to suggest he may force one because of a "dysfunctional" Parliament.

September's byelection results may give both leaders a sense of how they might fare in a general election. In previous byelections, the Liberals have lost two major strongholds, including the Quebec riding of Outremont.

But they have also held on to the Toronto ridings of Toronto Centre, which Bob Rae easily won, and Willowdale, which was taken by Martha Hall Findlay.

There are 308 seats in the House of Commons, including four that are vacant. The Conservatives currently hold 127 seats, the Liberals 95, the Bloc Quebecois 48, the NDP 30 and independents have four.

With files from The Canadian Press