Here's my current list, taken from two different sources (I only included cases with at least 4 kills):
>Flores-Daniels gang - 4 dead cops (1857)
>Juan Cortina & gang - 4 dead cops (1859)
>Cherokee nation - 8 dead cops (1872)
>Horrell-Higgins family - 4 dead cops (1873)
>Lee gang - 4 dead cops (1885)
>anarchists - 8 dead cops (1886)
>Sitting Bull, Crow Foot & Lakota tribe - 6 dead cops (1890)
>Gung Ung Chang - 6 dead cops (1898)
>Robert Charles - 4 dead cops (1900)
>Mill Reynolds - 6 dead cops (1902)
>Unknown - 9 dead cops (1917)
>Prison riots in Colorado - 8 dead cops (1929)
>Paul, Harry, and Jennings Young - 6 dead cops (1932)
>James O. Neely, Victor Lloyd Everhart and Carl Austin Dewolf - 4 dead cops (1946)
>William and George Bell - 4 dead cops (1948)
>Puerto Rican nationalists - 9 dead cops (1950)
>Bobby Davis:& Jack Twinning - 4 dead cops (1970)
>Prison riots in New York - 7 dead cops (1971)
>Mark Essex - 5 dead cops (1972-73)
>Branch Davidians - 4 dead cops (1993)
>Timothy McVeigh - 8 dead cops (1995)
>Al-Qaeda - 72 dead cops & 367+ more dead cops from 9/11-related illnesses (2001)
>Lovelle Mixon - 4 dead cops (2009)
>Maurice Clemmons - 4 dead cops (2009)
>Micah Xavier Johnson - 5 dead cops (2016)
Am I missing anything? I intend on going all the way back to 1776 and starting there, but given a lack of information and lists that aren't that complete and leaves several out. Byt so far I was able to find 25 cases since the mid-19th century. All that I do know ithats complete is every case that happened after 1945 (to which Canada only had two incidents of four or more cops killed that happened during that same period, namely Justin Bourque and James Roszko)