I have guppies and neons together and they do fine. In the same tank I also have harlequin rasboras, cories, and ottos. Of course, my tank is 29 gallons so I can have a few more fish.
You say you have 4 guppies (male and female). If you wait you will have more fish than you will know what to do with. You could probably add 4 neons without a problem and they would probably keep the guppu count at 4 by munching on the fry.
You say you have 4 guppies (male and female). If you wait you will have more fish than you will know what to do with. You could probably add 4 neons without a problem and they would probably keep the guppu count at 4 by munching on the fry.