Hydroxyurea (HU) dosage for CML initial use to bring down a high WBC (usage debatable -- more about that later) is done by body weight, about 25 - 30 mg per kilogram. The weight used is ideal weight, not actual if overweight. So if you are about 5'10" ideal weight for 18 yr old is under 156 lbs (71 Kg) then 30 x 71 = 2130 max per day. So I would very seriously question the 4000mg per day, which is about double the max recommended dosage. See, math is useful after all, and you slept through it this past year in high school.
Beyond that, Hydroxyurea (HU) for initial use to bring down WBC rapidly is a debatable issue. Before the TKI drugs (Gleevec, etc) existed, this was standard practice. Now that TKI drugs exist, and are considered less toxic than HU, I do not see the purpose in using an actual chemotherapy drug like HU to bring down WBC initially. The HU does not target leukemic cells, but rather kills all types of blood cells randomly, both good ones and leukemic ones. TKI drugs target only the leukemic cells. So it is the difference between using a shotgun (HU) and a rifle (TKI drug). I personally believe this killing of the good blood cells by HU is counter-productive in treatment of CML. It can lead to blood counts which are too low after the TKI drug starts working, and it is more toxic than TKI drugs.
You indicated your initial WBC was 300K, which is somewhat higher than average at CML diagnosis (CML average at diagnsois is under 200K). So your Onc is trying to bring down the WBC rapidly using HU. But the TKI drugs do the same thing, and as I see it, they do it more safely (less toxic). Again, HU is actual chemotherapy, and TKI drugs are not. For me, I would prefer to only use TKI drugs from the beginning. They do a fine job bringing down the WBC in a more controlled and safer manner.
So there are two issues: 1) the dosage seems way too high, 2) the use of HU at all is questionable.
I realize this is a lot for someone who should be only be considering what shoes to wear in the morning while skateboarding. But here you are with the rest of us, and we will help all we can.
So ask your Mom and Onc about this issue.