Even same species females could look a little bit different from each other, – some could seem more slender, and another more ‘thick’ – theres also individual differences.
It could also – quite likely – be young fish, and therefore not fully developed yet. And for sure males can ‘hide’ their sex and look totally blank, so there could still be a male in your four fish.
I have had several times where it has been a real nuisance that they only had same sex fish in the shop, – normally I think its males. I had some at one point, I think I had 12 males and 1 female 🙂
But if you cant get any more fish, then its really a nuisance. Thats one of things that often is a problem with licorice gouramies in shops.