If you can factor an expression then it's best to do so, as it makes the expression a bit more "use friendly." The expression (x+4)(x+1) tells you right away what the two roots are (-4 and -1), as opposed to x^2+5x+4. Also it makes it easier to see that there is no more simplification possible - suppose that for this problem the numerator had turned out to be 7x+28, which you can factor to 7(x+4); with the denominator in the form (x+4)(x+1) you can see right away that the (x+4) terms cancel, whereas leaving the denominator as x^2+5x+4 would make it less obvious.