Is it okay to love a girl who is 4 years younger than me , if she knows it ?

So I don't know if I can be in a relationship with a girl 4 years younger than me, and My conscience keeps reminding me that she is younger and become a sister automatically...!:!!? Even I don't have any comprehension for my logic for that age gap. But she keeps giving me hint IYKYK. And I don't know if it's okay ?? Btw I'm 25 and she's 21