Prerequisite for axapta 4.0 installation

For AOS(Application Object server)

Windows 2003 Server (including SBS)
Hotfix KB913184 should be installed.

Windows XP Professional (SP2+) XP (Production excluded)

For Ax Client

Operating system
Windows Vista all versions (SP1)
Windows XP Professional (SP2+)
Windows XP Tablet PC Edition (XP2+)
Windows 2003 Server (including SBS)
Windows 2000 Server (SP4+)

For Database server 

SQL Server 2005 (SP2 is recommended for performance)
SQL Server 2000 SP4 and higher
Oracle 10G Release 2 Enterprise Edition

You can visit following website to get more details.

Ax 4.0 hardware software requirement