While a lot has taken place between week one and week three, the fourth week is more critical because the embryo will stop attaching on the placenta and begin feeding through the umbilical cord. Besides, it is in the fourth week and fifth week that your pregnancy can be confirmed.
In the fourth week, the embryo (embryonic disc) will grow from 2 mm to about 6 mm. The process of internal organs development (heart, blood vessels, and others) that started in the third week will continue more rapidly.
As the embryo continues to grow rapidly, the mother also experiences extensive changes every day of the week. If you are very curious about taking that pregnancy test, go ahead, and it will confirm that you are indeed pregnant. However, the ideal time is the following fifth week when all the pregnancy hormones can be detected in the urine.
One thing that you will note during this week is that the changes may appear like they are becoming too much. Within just four weeks, you are vomiting, very high headed, and moods keep swinging from one moment to another! Well, take it easy, these are temporary and will go away when the first trimester goes away.