Question: I am using asp.net 4.
i wonder if it will be secure to caching sensitive data?
is it dangerous in manner of security or any other security issues?

Answer: 
It depends what you do with it. The data is held in the memory of the web server. There would be no way to access this data, unless you wrote a way to do it, or the attacker had access to your source code & write access to the web app.
I tend to cache the encrypted data and only do decryption at the point of request. Finally zeroing out the memory.

Guess if it was left in memory, in plain text, it would not be too hard for an attacker to work things out from a memory dump.