It's a significant feature of Chinese to use four characters as a whole in written language. In this way you can make your writing delicate and profound. Actually many of those four-character phrases tell a fable or an interesting story. 纹丝不动 is much better than 一动不动

yes,the latter one sounds more literary,while the first one is more colloquial

有区别，在口语中通常使用“一动不动”，写作中可以使用“纹丝不动”，而且纹丝不动具有夸张的意思和稳定的意思

有区别! “纹丝不动”意在形容“稳”，而“一动不动”是一种静止的状态。

一动不动比较口语化，纹丝不动比较书面化