* November synthetic quoted at $4.50/bbl over WTI
* WCS quoted at $15.20/bbl under WTI
* Suncor maintenance due to end this month
CALGARY, Alberta, Oct 5 (Reuters) - Canadian cash crudeprices weakened for the fourth straight session on Friday asproduction from the Alberta oil sands looked set to increase fornext month.
Light synthetic crude for November delivery was last quotedat $4.50 a barrel over benchmark West Texas Intermediate, downfrom $7.25 a barrel over WTI on Thursday, according to ShorcanEnergy Brokers. The premium has shrunken by $7.50 since Tuesday.
Western Canada Select heavy blend for November fetched$15.20 a barrel under WTI, compared with $14.20 under a dayearlier.
Supply constraints had been one factor pushing up Canadiancrude prices for the past month and a half, a trade source said.Suncor Energy Inc is in the midst of a six-weekturnaround on a vacuum tower at an upgrader at its northernAlberta oil sands operation.
That cut production to 300,000 barrels a day in Septemberfrom 373,000 the month before, though the work is scheduled tobe finished in the coming weeks.
Meanwhile, weather-related problems cut production atSyncrude Canada Ltd, to 318,900 bpd from 359,000 bpd.
Some refineries have also started maintenance recently,cutting into demand. The 362,000 barrel Wood River, Illinois,refinery run by Phillips 66 and Cenovus Energy Inc, began planned work last week.
On Tuesday, local media reported Exxon Mobil Corp's60,000 bpd Billings, Montana, plant began work to replace partof a fluid catalytic cracking unit.
(Reporting by Jeffrey Jones; Editing by M.D. Golan)
Keywords: MARKETS CANCRUDE/