It seems to me that this is one of the questions that come up from time to time. I know that I have posed it at one time.
There are other equally important considerations in addition to grain and sharpness. Those are the tonality of the print and the local contrast that the print exhibits.
Even if one were able to enlarge to 30X with no apparent grain in the print, it would still suffer from loss of local contrast and loss of smooth transitions in tonal scale as the degree of enlargement increases. The reason being that the negative can only contain so much information in a given film area. As we go larger in enlargement we "stretch" those areas over the print surface.
I personally enlarge most of my 4X5 negs to 11X14 size (maximum). Occasionally I will enlarge to 16X20 but that depends on subject matter etc.
There are different choices in developers that may enhance your ability to enlarge to a greater degree.