GH has several units in use, and gpg is one of them. The hobby uses ppm (parts per million, which happens to be equal to mg/l) and dGH (degrees). We can convert. A gpg of 4.2 equates to 72 ppm which equates to 4 dGH. So this is soft to very soft water.
That makes your options for fish must broader, as there are far more smaller-sized species in very soft water than hard water. Almost any fish from South America or SE Asia are suited, parameter-wise, so look at the smaller-sized fish. Ember Tetras, Neon Tetras or the rarer Green or False Neon, Black Phantom Tetra (since you had Black Skirts, these are similar in colour but very, very peaceful and not active swimmers), or some of the smaller rasboras from Asia... .With sand one of the dwarf cory species would be nice.