At week 4 of pregnancy, the ball of cells is splitting into an embryo and placenta. Baby's neural tube, the building block of the spine, brain and backbone is already formed. The amniotic sac and fluid which are forming into protective cushioning for baby.

How big is baby at 4 weeks?

At 4 weeks of pregnancy you baby is smaller than a poppy seed. Snd your baby is now known as blastocyst, a teeny tiny ball of cells, which is busy in settling into their new home (your uterus) prepping for all the crucial development which will occur over the next six weeks.

Pregnancy Symptoms at Week 4

The similar kind of pregnancy hormones which gives you positive pregnancy test can also cause some of the more typical 4 week pregnancy symptoms. These hormone levels increases quickly, so as it's normal to be 4 weeks pregnant with no symptoms, brace yourself for nausea and vomiting which will be in the near future.

Here are the symptoms during 4th week

Bloating

Mild cramping

Spotting

Mood swings

Morning sickness

Fatigue

Sore breasts

Source: https://www.thebump.com/pregnancy-week-by-week/4-weeks-pregnant