At four weeks pregnant, it is completely normal for the bottom of your tummy to not feel hard. During the early stages of pregnancy, your uterus is still quite small and hasn't expanded significantly yet. It usually takes several more weeks for your uterus to grow larger and become noticeable. So, there's no need to worry if you don't feel hardness in your lower abdomen at this stage. Every woman's pregnancy journey is different, and physical changes can vary from person to person. If you have any concerns or questions about your pregnancy, it's always best to consult with your healthcare provider who can provide personalized advice based on your specific situation.