Is it possible to already be experiencing some hardness and tightness around waistline at around 4 weeks pregnancy?

The waistline can: start expanding quite quickly. That could cause clothing to feel tight on you.

Possibly: Your body is preparing to accommodate your baby. Although at 4 weeks the baby is far too small to constitute to significant hardness or fullness, other issues may need to be ruled out - Such as a urinary tract infection. Please see your doctor for more information. Good luck