不 changes tone depending on where you find it. Here are examples:

When 不 comes before a 1st tone, it stays 4th tone

-你不应该给乞丐钱 - you shouldn't give money to beggars 

-我不吃鱼 - I was speechless

When 不 appears before a 2nd tone, it stays 4th tone

-他不学中文 - he doesn't study Chinese

-我不寻找你 - I'm not looking for you

When 不 appears before a 3rd tone, it stays 3rd tone

-我不想去中国 - I don't want to go to China

-我不喜欢喝茶 - I don't like to drink tea

-我不养猫头鹰 - I don't raise owls

When 不 appears before a 4th tone, it changes to 2nd tone

-我不是老师 - I am not a teacher

-不用谢 - you needn't thank me

-我不去 - I don't go / I'm not going 

It's strange at first, but you will get used to it the more you read.

if the following character is in tone 4, 不 should pronounce as bu2

eg.

不去 bu2 qu4

不用 bu2 yong4

不谢 bu2 xie4