Microsoft recently posted a blog listing the top 10 issues received through their support channel.  The issues relate specifically to the Outlook client for CRM.  If you have not read this, I highly recommend it.  Here is a short compilation of the 10 listed:

1. Outlook hangs during initialization (progress toolbar displays “Loading…” and never finishes)
2. With E-mail Auto Promotion Enabled, Outlook may not close cleanly/properly.
3. Contacts with birthdays earlier than 1/1/1970 causes Outlook to crash
4. Items in Shared Calendars will report Sync Issues
5. Cannot Promote an E-mail where the name of the attached file contains a ‘&’ symbol.
6. Selecting Dismiss on Outlook Calendar reminder for Service Activity causes record to change Status Reason back to default value for that record.
7. OWA Sent Emails Get Stuck in Outlook Draft Folder When Outlook Is Open And CRMADDIN is Enabled
8. Microsoft Outlook stops responding when you open, close, reply to, or forward e-mail messages in the Microsoft Dynamics CRM 4.0 client for Outlook
9. CRM Outlook client and/or Desktop Client hangs in presence of other Outlook add-ins
10. Phone call activity due time is changed after the activity is snoozed or dismissed.

Here is a link to the post: CRM4 Outlook Client – Issues and Fixes

For a list of all the hotfixes currently available for Microsoft CRM 4, check out the following KB article: Microsoft Dynamics CRM 4.0 updates and hotfixes