36week 4cm with #2 baby how far is labor!?

Hi I'm 36weeks 6day and 4cm with my second son L&D nurse say that with my second baby I could be 4cm for weeks but the say it actove labor! Not sure what contrations a this point feel like iv had pritty bad back pain grion hurts bad and I can barely walk I get menstral like cramps and a sharp pinch on the lower left side of my back!!! Kinda scared of when to know when it time cuzz my frist I dint have any problems tell day of brith!!! What should I do and what will happen!!?? What is 4cm really mean!!?? Thank you