Don't worry - I'm probably not explaining it well!
The Client Lookup will get you the Client name field (which I think you are using for the client nuber) and Client Unique Identifier(GUID)
To display another field (other than the standard field shown on the lookup) you will need to make a request to the server to get the other data. This can be done in the following ways:
1. Use a Relationship mapping on the Client->Matter relationship to populate the client name field on the matter form. This will only work when you create a matter from the client record - and if the client name changes or the client on the matter changes, the name will not be updated. (www.powerobjects.com/.../microsoft-dynamics-crm-relationship-mappings-help-keep-customer-and-bill-to-customer-consistent)
2. Within the JavaScript Onchange event of the Client field on the matter form, use the REST/SOAP end point to get the client name field and populate a text box on the matter form. This will work as long as the record is being updated via the form (not bulk update/import/workflow/sdk), and the client name never changes. To cater for this, you will also need a plugin/workflow to do the update server side.
3. Create a webresrouce on the matter form that looks up the client ID from the parent Xrm context and then gets the name using the REST/SOAP endpoint and displays it as HTML.
Option 3 is probably the easiest and least impact on your system, and is the subject of the link I provided - blogs.msdn.com/.../showing-related-information-in-a-crm-2011-form.aspx
hth