Over the last few major version changes, Learning Movable Type has published guides to upgrading safely, without risk of messing up your blog. The most recent tutorial was A Safe Way to Upgrade to MT3.3.
The three main steps that still hold for safely upgrading to MT4 (from MT3) are:
- Make a back up of your database.
- Make sure all the plugins you use for your site have upgrades that are compatible with the new version.
- Install MT into a new directory in your cgi-bin. Do NOT overwrite your existing MT directory.
The Movable Type ProNet community has done a stellar job in outlining everything you need to know to do a successful upgrade to MT4. These instructions can be found at the MT Community Wiki in the Community Generated Upgrade Guide.
Please see the instructions at the Community Wiki for the specific details on how to safely upgrade your MT blog to MT version 4.