Some time ago I wrote a post around how to show and hide fields based on the logged in user. Apparently this doesn't work for Dynamics CRM 4.0 anymore for two reasons. This post will help you to get it working again.
The first one is that you are not allowed to use the <%= approach anymore. You do now have to use the function:
The second reason is that the variable "Microsoft.Crm.Security.User.Current.UserAuth.UserId" does not exist anymore. Microsoft has changed the internal structure of how they work with the current user. You now should use this "Microsoft.Crm.Security.User.Current.SystemUserId.ToString()". These two changes lead to the following code
var loggedInUser = '<%Page.Response.Write(Microsoft.Crm.Security.User.Current.SystemUserId.ToString())%>';
If you do have a single mistake in the aspx page you are changing, then you'll get the following error:
The VirtualPathProvider returned a VirtualFile object with VirtualPath set to '/MicrosoftCRM/sfa/accts/edit.aspx' instead of the expected '//MicrosoftCRM/sfa/accts/edit.aspx'
This has nothing to do with the VirtualPathProvider, it's just that you have a mistake in your aspx page.