SQL Server uses only 1 CPU of 4 available

I've two machines with SQL Server 2012 Express SP1 with default configuration settings for CPU usage.

I'm using this query on my work machine:

SELECT cpu_count
FROM sys.dm_os_sys_info

It returns "4", which is true.

The same query on production machine returns "1" (that machine has 4 processors too).
There's no additional CPU usage settings in both cases, except defaults.

Production machine is a virtual machine, which, possibly, was reconfigured after SQL Server installation.

Is there any way to reconfigure SQL Server for using of 4 processors?

SQL Server Express is limited to 1 CPU only

(to add more details it's limited to one socket or four cores, whichever is less)

see http://msdn.microsoft.com/en-ie/library/cc645993.aspx

StackExchange Database Administrators Q#39068, answer score: 10