Hey Kathy,
I wouldn't worry too much, like what Pat said...The smaller, the better....Even though 4cm seems pretty big, there are some thymomas that are 15cm! Mine was 5cm x 5.5cm x 7cm

I read some study that said that when the tumor is smaller than 5cm it's less threatening just because it's more likely to be encapsulated, and therefore hasn't spread beyond the thymus--which usually means it's benign! Don't worry too much, girl! You'll get through it!
I know what you mean about the age thing...That's one thing I keep tossing around in my mind...It says that the most common age to get a thymoma is between the ages of 40-60...So does this mean that my body has prematurely aged to that of at least a 40-year old, or maybe I'm genetically pre-disposed to get a tumor there more easilly than others? I do have other autoimmune problems, so I'm guessing I might have that gene that makes the immune system frail and bedazzled...It's sucky! But I'm planning on making my lifestyle as healthy as it can be when my body recovers...I've been smoking since I was 13, and although this isn't a risk factor for getting a tumor in the thymus, I'm sure it helps to get the ball rolling in the cancer depot...
I'm looking into other avenues as well...I read a study that said that there are thymomas with estrogen receptors (those ones actually have a better prognosis), which might make a case for a hormonal cancer, much like breast, prostate, reproductive...I'm planning on steering clear of xenoestrogens as much as possible from now on...I'm also prone to fibroadenomas (one of which the docs thought might have been cancer as well, but it was biopseed and found to be okay...it's been a rough year Eek!)...i know that breast lumps are exascerbated by caffeine and other xenoestrogen-containing agents...so bring on the decaf and phytonutrients!
Anyway, things will be okay...We have eachother to turn to for advice, relating and comfort! Let us know how things are going each step of the way and please take care!
Thanks, Connie! You will also do well because you are Connie!