What database are you running on 4.1.

If it is sql 2005 express than you may have some problems.

MS SQL Server 2005 Express

supported :No

After you upgrade to a database server that is supported by vCenter

Server, you can install or upgrade to vCenter Server.

MS SQL Server 2005

supported:Yes

You can install or upgrade to vCenter Server.

MS SQL Server 2008 Express

supported:No

You can perform a fresh installation of vCenter Server 5.0.

You cannot upgrade because vCenter Server 5.0 is the first release that.

The above is a cutout from the PDF that VMware has for upgade path, if I understood it well.

Hope it helps.