Yes you should be able to get a positive result on a urine pregnancy test at 4 weeks if you're pregnant.
However If your pregnancy test is negative and your period is late, then you should wait a week before taking the pregnancy test again.
Pregnancy Symptoms usually begin at 4 weeks and one clear sign that you may be pregnant is a missed period.
Other symptoms of pregnancy are Tender breasts, Swollen Breasts, Morning Sickness, headaches, constipation, mood swings, cravings for food or food aversions, nausea, vomiting and slight cramping or slight bleeding.
Those are signs of pregnancy and if you have implantation bleeding you're likely pregnant.