When developing new workflow activities for CRM 4.0 you need to release the filelocks on the dll to be able to copy the new dll to the assembly directory. This can of course be done manually by restarting the service in the service manager but can be scripted into the prebuild events aswell, so that it is more automatic. Add the following two lines to the Pre-build events of your workflow activity project to make it restart the MSCRMAsyncService:
net stop "MSCRMAsyncService"
net start "MSCRMAsyncService"
Please note that you might inflict any ongoing workflows by doing this so take care if it is a live production server.
Microsoft Dynamics CRM Architect