To clarify my above statement. I only elarge by 4x by area. Thus 4x5 negs get enlarged to a max of 8x10. My 8x10 negs get enlarged to a mx of 16x20. However, my 8x10 negs make great 11x14 prints too. There's a noticable difference between an 11x14 from a 4x5 neg vs an 8x10 neg.
I've noticed that some people consider a 4x enlargement to be the increase in length along one side. e.g. 4x5 at 4x becomes 16x20. This is a 16x enlargement by area, which is too much IMHO.