Eh, don't worry too much lah, spotting dengan cramps at 4 weeks boleh jadi normal je. A few of my mum friends had similar experience, especially if the blood is light pink or brownish (that means old blood, not fresh). Could be implantation bleeding, which is just the baby settling in, or just your uterus doing its stretching thing.
Lepas that, the cramps usually calm down after a few days. BUT saying that, every pregnancy is different, better safe than sorry, so do check with your doctor or specialist to confirm everything is okay, especially if the bleeding gets heavier or the cramping gets really bad. Fingers crossed for you!
Norsha