They have said for years that it is much harder for women with Cf to conceive. That being said, there's tons of ladies on here who have either had no trouble getting pregnant naturally, or had surprise pregnancies. Cf is pretty much completely different for each patient. And the fertility issue is different for each woman. Unlike men with Cf whom the majority of have fertility problems.
I don't know anything about fertility post transplant.
There are no signs of pregnancy 4 days post conception. The earliest you would have symptoms would be 10 days and even that is stretching it, you'd have to really be looking for them or have already had pregnancies to know how it feels for you.
The common early pregnancy symptoms are very sore breasts, hunger, bursts of energy, waves of total exhaustion, and menstrual type cramps. Sometimes around day 10-12 post conception you'll feel crampy and think oh I guess I'm getting my period after all, but its actually pregnancy cramping.
The earliest you could take an at home test would be 9 days post conception, and the accuracy of the test is around 55% I believe. There is almost 99% accuracy 14 days post conception. The most accurate test would be a blood test, but this still can't be done until 14 days post conception.
I know you are obviously very worried. Please remember that whatever comes-everything will be alright!