If a man is married to four women, one of which dies, is he authorized to marry a fifth one?
Islam allows a man to have four wives at any time, only if he can do justice between them, and any one of them is not neglected (in terms of worldly needs, affection etc.). This does not mean that one can only marry four times in his lifetime.
This is translation of Yusuf Ali.
If ye fear that ye shall not be able to deal justly with the orphans, Marry women of your choice, Two or three or four; but if ye fear that ye shall not be able to deal justly (with them), then only one, or (a captive) that your right hands possess, that will be more suitable, to prevent you from doing injustice. —Quran, Sura 4 (An-Nisa), Ayah 3