THE DUST-JACKET of Cold Blood IV (Mosaic, 242 pages, $19.95 cloth), edited by Peter Sellers, depicts a veritable pantheon of classic detection: Holmes, Marple, Poirot, Wimsey, Father Brown, and - the sole colonial - a Spade-Marlowe amalgam in the likeness of Humphrey Bogart. Apart from the presence of a trunk labelled "Toronto," I could deduce no plausible reason for these sleuths` appearance on the cover of a collection of recent mystery stories by Canadians. In fact, few of the stories in this anthology adhere to either the amateur/country-house tradition or the gumshoe/mean-streets model, and only about half of them even deal with the question of who done it. There are two convincing cc period" mysteries, Ted Wood`s "Murder at Louisbourg" and Charlotte MacLeod`s "Why the C Was Boiling Hot," set respectively in 18thcentury Cape Breton and l9th-century New Brunswick. Three writers use the Caribbean as their setting: Gregor Robinson, whose "Coup d`Etat" is a wry account of the perils of office politics in a tropical bureaucracy, Alison Cunliffe, whose "Plumbing the Depths" plunges its victim into deep water off the Cayman Islands, and Anne Stephenson, whose "Bermuda Short" sends a scorned woman on a vengeful vacation, straining credulity a bit in the process, and employing a murder method that while of recent vintage, seems already a trifle tired. On the other hand, "Anna Said.... Peter Robinson`s excellent Inspector Banks story, dismantles its love triangle with a very contemporary and highly appropriate weapon that, we are told, also has historical precedents. If this collection does have a predominant theme, it`s the sheer unpredictability of dreadful events. William Banker`s "The Best of Birtles" and Eric Wright`s "The Lady from Prague" expertly string readers along until the final twist, and Peter Sellers`s "Last Resort" has an ambiguous ending that suddenly calls all previous assumptions into question. In John North`s "Double Bogey," unexpected complications turn insurance fraud into murder, and in Jas. C. Perrin`s "A Murder Story," a mild-mannered clerk discovers his untapped potential for mayhem when he`s abducted by a goofy, if implausible, serial killer. The collection`s weakest links, to my taste, are Mel D. Ames`s "The Disappearance of Sarah-Sue" and Eliza Moorhouse`s "Night of the Fourth Moon," both of which, though they are quite different, adopt a style that struck me as simultaneously arch and corny. Regrettably, the whole book is plagued by a highly distracting lack of copy-editing; "lay" for "lie," "pour" for "pore," planes that "take-off," and that famous Massachusetts clan, "the Kennedy`s" (to mention but a very few): these are not the least of the crimes committed in Cold Blood IV.