As of September 30, 2013 the only versions of Sage 100 ERP which are supported by Sage are:
Sage 100 ERP 2013
Sage 100 ERP v4.5
Sage 100 ERP 4.4
Version 4.3 of Sage 100 ERP (formerly MAS90 and MAS200) will no longer be supported. This means those on version 4.3 (or earlier) will not get access to any product updates which might fix bugs or ensure compatibility with Microsoft releases of Windows 8+.
Also please remember that Sage will no longer issue payroll tax form updates for any version of Sage 100 ERP and instead you will use the included Aatrix tax functionality to print forms which is only available for supported versions of Sage 100.