In a cover band, rhythm guitar=one more person to split the money with. Getting enough for 4 to get paid is hard enough, 5 is impossible most times in this area. Unless the singer plays rhythm, they are kinda rare.
If you have four including a rhythm guitarist, there is no need for a lead singer. Between the bassist, rhythm and lead guys, at least one person should be able to sing. The rhythm guy perhaps can sing over his lines, or the bassist can, or both of them can. Or the lead guy can sing when he (or she, as it may be) isn't playing lead.