ratheeshpv,

Mitchell has posted a blog regarding an upgrade such as yours, you can view it here.

HTH

As with any upgrade (DNN or not), certain precoutions need to be taken. This will allow for an easy 'rool-back" if needed. You should have a back-up of both you DNN database as well as the files in your web directory. This can not be stressed enough. Another thing to check is any custom modules that you have installed/built. If you have purchased any third party modules from other developers (from say the store at Snowcovered), I would check to see if they are DNN 4.x compatible. Be certain also to check the 'patch' area for upgrades that may be available (if your logged in to Snowcovered, it's under your account information).

Looking at Mitchells instructions paint a pretty clear picture as to the process...your first move should be to upgrade (incrementally) to v3.3.5 or higher. I would back-up and test at each stage of this, then again before making the jump to v4.x.x.

Following the back-up listed above (and detailed on Mitchell's site) will enable to bring your "old" site back online with no ill effects both quickly and easily, in the event something goes wrong.

HTH