Here the typesetter for the 1841 British edition omitted the definite article the before “forty and first”. In this case, the LDS text has maintained the shorter reading up through the present edition. Note that other numbers in this passage have the the before the number: “the thirty and eighth year … the thirty and ninth … the forty and second”. We get the same use of the later on in this verse as well as in verse 14:
The critical text will restore the original the before “forty and first”. For a similar loss of the before a number (in this case, in the 1892 RLDS edition), see nearby under Mormon 3:8. It is possible that the loss of the the was influenced by the fact that the text sometimes refers to the passing of time by specifying a set number of years, in which case the the is not used (as in 4 Nephi 1:6: “yea even until forty and nine years had passed away”).
Summary: Restore the definite article the before “forty and first” in 4 Nephi 1:6; the current reading without the the is quite odd since all the surrounding references to a specific year have the the.