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: 
As per Microsoft's pattern & practices this what they suggest

[Do not cache sensitive data](http://msdn.microsoft.com/en-us/library/ff650867.aspx)

> 
> If your service method contains data that is sensitive, such as a
>  password, credit card number, or account status, it should not be
>  cached. If sensitive data is cached on the client machine, it has
>  serious security implications because it leaves interesting data
>  available to attackers.
> 
> 
> Perform the following steps to ensure that sensitive data is not
>  cached:
> 
> 
> Review operations for sensitive data. Review all of your operations
>  for usage of sensitive data. This could include but is not limited to:
>  Information that either contains personally identifiable information
>  (PII) or can be used to derive PII that should not be shared with
>  users Information that a user provides that they would not want shared
>  with other users of the application Information that comes from an
>  external trusted source that is not designed to be shared with users
>  Review the operations for caching of sensitive data. Review how each
>  operation manages sensitive data and ensure that it is not cached.
>  There are three patterns of sensitive data caching that you can review
>  for: Custom caching code such as use of a Dictionary or SortedList
>  object Use of the ***ASP.NET cache via System.Web.Caching.Cache***. Use of
>  an Enterprise Library caching block
> 
> 
>
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.