Period 4 days late, negative hpt, is my hpt reliable?

I've had mild cramping on and off for about 2 weeks now, and my period is 4 days late. I've taken a few hpt's and they've all been negative. I bought them off ebay they are supposed to be the early detection ones.. are these ones reliable? Should I test again in a few days or should I just forget about it? Thank you so much!