Sometimes could 4 weeks be too early to tell your pregnant?

Please clarify: I don't understand the question. Four weeks after what? Two weeks after ivulstion, if a period is missed, then testing should be accurate.

Both urine ; blood: Pregnancy testing should be able to detect pregnancy four weeks after episode of vaginal intercourse.

Yes: Definitely if your last menstrual period was 4 weeks ago exactly, if you ovulated late in the cycle, it could be too early to tell if you're pregnant. If you're worried, wait 5 days or so and re-test. Best of luck.