If a gene has 4 exons how many proteins can be made

Before answering the question, you should know that exons/intron systemin the gene exists only in eukaryotes. A gene can have many exons with introns between them, but in RNA maturation, these exons will be joined together to form one RNA to code for one protein (There's no relation between exons number and how many proteins are formed).

In prokaryotes it is different there's no exons/introns, but their gene can be transcripted to a polycistronic RNA which can give after translation more than one proteins.