S.A.
V.,
I am so sorry to hear about your diagnoses but glad to see that you are facing it head on and looking for resources for you and your daughter. Know that the prayers of my family are with you in this journey!
Regardless of how smart your little girl is intellectually, remember that emotionally she is still 4 years old. Mommy being sick is a scary thing for one so dependent on you, so being too realistic will frighten her. There is a fine balance with kids this age - she is still very literal and doesn't understand that she can't "catch" what you have - so, talk of germs may not be a good move. And like all things for kids, address each question as they come up and be willing to tell the same information again and again and again.
Personally, I would wait until you have all of the information that you need prior to talking to your daughter. Be honest about when you go to the Dr. but wait for the "big talk" until you know more details about what will happen and when and what plans you have for her. That way, you are ready to answer all of her questions.
Also remember that your child will "play out" her feelings - kids aren't big sit and talk types but you will see her emotions playing out in her play - she might play Dr. more and if you listen in to what she is telling her patients (stuffed animals, etc) then you will be able to gage what she is thinking about your illness and treatment. I would also NOT make her responsible for more as someone else suggested... it is a mistake to demand more of her "so you need to help Mommy" or "be a good little girl" or "take care of brother" - this is a huge thing for your child (and everyone in your family) and to put more responsibility on her at this time is unfair and she will learn to resent you and the illness for putting that on her. Any cancer treatment is a long journey and she will act out and get frustrated and she needs to know that its OK to express her anger and frustration.
Other than that - if you don't already have a spiritual home, now is a great time to find one. There are several great Churches in the Sugar Land area (if you are Christian) that have supportive members and clergy who will be a big help to your family during this time.
AND, I know an amazing woman in Sugar Land who is a breast cancer survivor - her kids were older when she went through it but I know that she would be willing to talk with you if you would like. I will try to find her contact information for you as I haven't talked with her in a while. She beat cancer and that was more than 10 years ago.
blessings,
Stacy