This is what I have for April record lows across northern tier states (as well as southern Canada)...corrections welcome.
Washington: -7 Lake Keechelus 4/1/1936
Oregon: -23 Meacham 4/1/1936
Idaho: -22 Copper Basin 4/5/1997
Montana: -33 Gates Park 4/2/2002
Wyoming: -29 Lake Yellowstone 4/1/1920
North Dakota: -24 Powers Lake 4/2/1975
South Dakota: -16 Deerfield 4/3/1951
Minnesota: -22 Karlstad 4/6/1979
Wisconsin: -17 Rest Lake 4/1/1923
Michigan: -17 Champion Van Riper Park 4/4/1954 (official), Bergland -34 4/1/1923 (unofficial)
New York: -24 North Lake 4/1/1923
Vermont: -13 Mt Mansfield 4/7/1982
New Hampshire: -20 Mt Washington 4/5/1995
Maine: -14 Clayton Lake 4/2/1964
BC: -31 Smith River 4/1/1954
Alberta: -38 Peace River 4/2/1954
Saskatchewan: -40 Key Lake 4/1/1996
Manitoba: -35 Port Nelson 4/14/1923
Ontario: -35 Hornepayne 4/2/1932
Quebec: -35 Doucet 4/1/1923
FWIW I still consider the -34F at Bergland on 4/1/1923 the official state record low for MI for April, though I understand the skepticism. I'm certainly open to the idea that this reading was not taken with a properly calibrated/sited thermometer. Outside of that reading - Watersmeet, MI fell to -20F on 4/7/1982 (Champion Van Riper Park fell to -16F & Ironwood and Kenton both fell to -12F that day so I would say that reading is corroborated). Champion Van Riper Park also hit -17F on 4/10/1997 in addition to 4/4/1954. I also have a remarkably late -16F on 4/10/1909 at Lac View Desert, MI.
Rest Lake, WI supposedly fell to -20F in April 1924 but their daily summary stats show -17F on 4/1/1923 as the monthly record, which is what you have. The monthly state records table @ Infoplease has the -20F reading:
Upon closer inspection this appears to be a typographical error. Unfortunately those Infoplease tables are full of errors and inaccurate/unverifiable readings. The original COOP form for April 1924 shows -15F on the 1st. Its pretty remarkable that Rest Lake was that cold exactly a year to the date after 4/1/1923, but no -20F reading can be found:
Interestingly enough, Stanley, ID hit -30F in the same cold wave (on 3/30/1924). Just two days later and this reading would have blown away the official state record low for April of -21F in 1936 @ Alpha.
Wyoming has had a couple close calls with the official state record of -29F @ Lake Yellowstone on 4/1/1920. Old Faithful was -28F on 4/5/1936 and Moran was -28F on 4/3/1945.
Dubois, WY hit a ridiculously late -13F on 4/30/1909. This may be a legitimate reading as Cheyenne was 10F that morning and Dubois is a colder location.
For South Dakota, Deerfield 4NW has a reading of -22F on the books on 4/2/1975. I notice you have -16F as the state record low for April? I also have Rochford with a ridiculously late -15F on 4/13/1997.
For Minnesota, also -22F at Tower on 4/6/1982. And an incredible -32F on 3/31/1975 at Tower (corroborated by -24F at Gunflint Lake 10NW that morning).
For Maine, also -14F @ Squa Pan Dam on 4/2/1964. The Clayton Lake reading appears to be from 4/1/1964, not 4/2/1964 as you have listed. I see Clayton Lake has also hit -24F on 3/29/1974.
For Alberta, I also have -38.9C (-38F) at Fort Vermillion on 4/4/1920. Identical to the -38.9C (-38F) at Peace River on 4/2/1954 that you have.
For Manitoba, I have a close call @ Brochet with -34F on 4/11/1961 and -33F at Wanless on 4/4/1936.
Also worth noting that Wabush Lake, Labrador has seen -35F on 4/7/1969.