ContainMultiline is used to perform a case-insensitive comparison against the content of a text file. ContainsMultiline does not strip end-of-line characters; to pass the test, the assertion must include correct end of line character sequences:

Set-Content file.txt -Value "1`n2`n3`n4" 
'file.txt' | Should -FileContentMatchMultiline "2`n3"