You cannot purchase Web Premium CS4 anymore! Shocked? Then here is a convincing answer for you.. :
Since you want to purchase a Volume Licensing, then you can purchase the current version which is CS6 and "Downgrade" to CS4. This Downgrade is available Only with Volume Licensing and doesn't come with Retail version. You can purchase the Volume Licensing of CS6 over Adobe Licensing site Or from a Reseller and contact Adobe Chat support or Phone support to request for a Downgrade.
Adobe issues only the serial number for CS4 after downgrade but doesn't not issue the download/box for that product. Hence before requesting for Downgrade you need to arrange the media of CS4 yourself. You can go to the below site to check if the required product is available for Download Link/ Replacement Box.
If it is not available in the above link, you can contact Adobe's authorized reseller to check with them if the media is available.
Once you purchase CS6 Full version and the media for CS4, you can contact Adobe Support to request for Downgrade. Usually they would ask for these queries, hence keep these information available when you contact them:
1. Downgrade Product name, version, platform and Language
2. Reason for Downgrade
3. End-User ID or Deploy-to ID(You will come to know these anonymous words when you purchase any Volume Licensing product)
4. Order number for CS6 and the Replacement media for CS4.
With all these information, contact Adobe Chat support, the issue should be resolved in your First contact if possible.