MS CRM 4.0 does not provide filtered lookup facility, but we have some couple of options that we can use to implement filtered lookup in ms crm 4.0 like
- Change lookupsingle.aspx page
2. Buy stunware filtered lookup tool
3. Using JS Code
4. Writing plugin.
You can choose option that suite to your requirement. But I hope we will have this features available in MS CRM 5.0 🙂