Yes, it is 100% safe and recommended
The warnings on my heating pad tell me to not put it on my stomach if I'm pregnant. I still use it when my back starts hurting but use it on the low setting.
I just discovered that propping two pillows under my head/shoulders helps my cramps so away while sleeping. And first trimester so perfectly safe! Hope this helps!
Also *go away ;) Forgot to mention I'm 4 weeks as well!
My dr said a heating pad is ok. She said if I start to feel uncomfortable bc its too hot then move it or turn it down. A bath helped me too.