Titus Welliver is one step closer to being officially single ... for the 4th time -- he's filing for legal separation from his wife.
According to court records ... Titus filed Tuesday in L.A. County Superior court to make it his split from Josepha Theordora Stemkens all legal. The couple's been married since April 12, 2014.
Titus, best known as the star of "Bosch," has also had big roles on "Lost," "Deadwood" and "Sons of Anarchy." This was his fourth marriage. He'd previously divorced Heather Wielandt and Joanna Heimbold.
He was widowed after Elizabeth Alexander died of breast cancer in 2012.
Welliver has a daughter with Alexander and 2 sons with Heimbold.