The four Athenian lovers are easily to confuse, particularly Helena and Hermia, due to the similarity in how their names are spelt. It seems as if Shakespeare deliberately present them as characters who are not easily distinguished from each other. From their presentation at the very beginning of the play, they are all characters who are defined by their love for another, and therefore it could be argued that Shakespeare uses them as an example of the vagaries of love in the human body. Once in the forest after all, they all quickly shift in their feelings and emotions towards each other, with even Hermia showing her true colours when both Lysander and Demetrius turn away from her and fall in love with Helena. Note what Hermia says to her rival in love in Act III scene 2:
I am not yet so low
But that my nails can reach unto thine eyes!
Hermia here shows herself to be one of the more extreme and violent characters now that her position as being the object of both men's affection has shifted.
In short, the four Athenian lovers are meant by Shakespeare to be confused. They are not meant to be distinguished easily, because they are used in this comedy to represent the ease with which human love is changed and how fickle it actually is. The characters have few distinguishing features apart from their changing emotions as a result.
In A Midsummer's Night Dream one can easily interchange the four young lovers—as the lovers themselves do. During the course of the play Demetrius and Lysander are, at various times, both in love with Hermia or with Helena. This illustrates Shakespeare's theme that love is a form of madness. When it comes to love, the normal criteria of reason short circuit—love has all the illogic of a dream and all the fantastic magic of poetry.
The four characters, however, do have some distinctive qualities. Hermia is shorter than Helena, allowing Shakespeare to pun on the meaning of low as both short and lacking in a moral stature. Hermia also comes across as having higher self esteem than Helena—she is more likely to fight back like a spitfire than to submit to being someone's spaniel to be kicked, as Helena says of herself in reference to Demetrius. Demetrius appears more fickle than Lysander in having changed his allegiance before the play starts from Helena to Hermia—though Lysander also, under the influence of the love potion, changes his spots to fall in love with Helena.
In his book A Theater of Envy, Rene Girard devotes six chapters to A Midsummer's Night Dream in support of his theory of mimesis of desire: Girard believes humans only want what other people want. We only fall in love with a person when we see somebody else has first, he argues. In his view, Shakespeare deliberately made the four characters interchangeable to demonstrate that it's not the personal characteristics of a lover that attract us but the fact that someone else desires him or her. Whether you agree or not, it's interesting to think about.