Dear, if the doctor said it was OK, then listen to him/her or seek another opinion. At four weeks, there is hardly much to see on the ultrasound (maybe what look like 4 to 12 cells - not really much) Wait until the next one, it will reveal a little more. At the beginning of the second trimester, it will be the size of a peanut, so you see how fast they grow, and will even be able to hear a heartbeet.
Get reassurance, and please don't let things stress you out. Check on the site from Rikj, and below are a few more, some even have forums where you can ask a doctor directly. BUT remember these do NOT take the place of a doctor's visit.
Good luck, and keep us posted.
http://www.mayoclinic.com/health/sym...gnancy/PR00102
scroll down and even get answers on flying while pregnant, exercise,
dangers, etc.
http://www.epigee.org/pregnancy/symptoms.html www.askdramy.com The U.K. online doctor.you can post on a forum, or 'ask the Dr. Amy' with a click. The FAQ Tab on the homepage is also a good place to click and has a lot of good subject matter. P.S. Most of the articles are printable in PDF format.
Just don't do any heavy lifting, or exercise that you are not used to already, and no saunas unless you've been using them at least twice a week for many years.

It's all hurry up and wait now, so take good care!