The only usual symptom that shows up in the first 4 weeks is the missed period. Weight gain, nausea, etc., don't usually show up until later (if at all, in the case of nausea). Generally 3,4 months for obvious weight gain. Anything earlier is usually a hormonal and appetite change because as Kwackers pointed out, there's hardly anything there yet. Mommy2An_Angel's statement that 'iloveryan is very wrong' is actually misleading because she herself said she was looking bloated at 6 weeks. So, still nothing in the first month.
Also, a lot of women gain weight (water weight) before their periods. Hardly a sign of pregnancy.