On what kind of platform are you? If you are on a VPS (Virtuozzo, Xen, or similar), which I assume you are, then you shouldn't try to upgrade anything at the OS, since the OS is managed at the host-layer of the physical server. If you want to upgrade, you should ask your host.
Even if you are on a dedicated server, be very careful when upgrading the OS from 3.x to 4.x if you have any software installed that relies on the OS' libraries, such as a control panel. Upgrading from 3.x to 4.x isn't really that difficult, but it's also very easy to break applications that depend on the OS.
The safest way to do such an upgrade is to ask your host to setup a new VPS (assuming that you are on VPS right now) with the OS you want, and then migrate your services over to that new VPS. Most hosts wil be able to do this at a low fee for you, as they will want to retain their customer.