You'll have to export your data, uninstall Unity 2.x, install stuff needed by 4.x (SQL for instance) and then install 4.x and import your data back in.
4.x will run with Exchange 5.5 - I can't speak to your hardware support, I don't know the specs of the box you noted. Likely you'll need to move Exchange off box if you have it installed on the same server with 2.x today.
You can check out the Upgrade and Migration Guide for 4.x for more details on this:
http://www.cisco.com/en/US/customer/products/sw/voicesw/ps2237/products_upgrade_guides_book09186a0080222fdf.html
