3rd pregnancy and am 4 weeks. Lots of cramping and low back pain. Not constant it comes and goes. Is this normal?

Can be. Cramping can occur at any point in pregnancy and can be normal. If not accompanied by bleeding, then it's typically okay, but can be disconcerting. If you're worried, can always see a doctor, get an ultrasound and make sure everything is okay. If continues may want to get baseline cervical length in pregnancy and follow. Best of luck!