Windows Installer 4.1 - This will be available with Windows Vista SP1 and Windows Server 2008. As of with the SDK this version will not add any new feature. By the way Windows Server 2008 is currently under beta and is expected to be released later this year.

Windows Installer 4.5 - One of the main goals in this version is to improve the installation of multiple MSI packages thus enabling products to split into smaller MSI files. To achieve this APIs are add which acts a chainer to call and install multiple package operation. In that case there will be install and rollback actions across these multiple MSI files.

What I understand is that the chanier itself will not be a part of MSI 4.5 reditributable instead will ask the tool vendors to support these new features in their product. The MSI 4.5 redistributables will be made available with the Windows XP SP2. This will include features and fixes from MSI 4.0 which is currently available on Windows Vista.