Darinbad beat me to the punch. In my haste I assumed that you are pretty much finished with older legacy projects that use A3. If you wish to go back to projects that are using A3 you WILL get prompted by Sonar that the plugin has been uninstalled or moved and you have to replace all the tracks in those projects with A4 and probably have to re-mix and re-master. That is because A4 is an all new program and not an upgrade!
BUT, if you are moving on and just concerned with using A4 on all projects from this point forward, you do have the option to install along with 3 so you don't have that problem. If that is the case, simply install 4 and restore gear from Custom Shop and BTW!!!! when I say Restore I mean from the Custom Shop logo on Amplitube 4s right hand corner. Not the standalone version of Custom Shop.
Please forgive me for overlooking the fact that you may want to still use A3 in older projects. I have been around for so long in the business that I tend to think old school. In other words, once an album is finished it goes out as is! There used to NOT be the option of going back and replacing just an amp sound without re-amping or re-recording! What you played thru was on there and after the release there was no turning back. You just had to muddle on until you could afford better gear and instruments.
I thank Darinbad for catching my mistake!!!!