Hello,
If you are referring to increasing the volume of a datastore you may use features such as volume grow from vcenter or add an extent to increase the volume which will combine another lun which is available to your host.
You can find configuration maxims at the below link for 4.0 which will give you a deeper understanding of . I also added another link from vmware which provides greater detail in the storage field.
http://www.vmware.com/pdf/vsphere4/r40/vsp_40_config_max.pdf
http://www.vmware.com/files/pdf/Storage-with-VMware-vSphere.pdf
I'm just curious but is there a specific reason you are just evaluating 4.0 and not 4.1 or 5?