hi there :)
Even though there is always a risk of pregnancy if you have unprotected sex, in this case i think it will be very unlikely, if you had sex 4 weeks ago and u haven't experienced anything out of the ordinary i would say that your just fine.
Pre-cum doesn't contain sperm, it will only ever contain sperm if the male had ejaculated before hand. I think in this case your pretty lucky but things you can look out for are tender breasts, mood swings or late/missed period, if these happenings occur then you should get a pregnancy test to be sure but in the end result its very unlikely that someone will fall pregnant just from pre-cum.
But u need to understand the seriousness of this ! if your not on the pill and not using a condom "pulling out" doesn't always mean its safe, even though pre-cum doesn't have sperm in it there is still always a chance ! if you dont want a pregnancy then use a condom or go on the pill !!
i hope everything turns out well .. best wishes :)