WF4并非是在原有WF3.x上的改进，而是全新的开发有新的命名空间，程序集，原WF3的部分会继续保留这主要是为了兼容，下面的表格告诉你具体的变化：
1.程序集的变化
| WF3.x程序集 | WF4程序集 |
| System.Workflow.Activities.dll | System.Activities.dll |
| System.Workflow.ComponentModel.dll | System.Activities.dll |
| System.WorkflowServices.dll | System.ServiceModel.Activities.dll |
2.名空间的变化
| WF3.x命名空间 | WF4命名空间 |
| System.Workflow.Activities | System.Activities.Statements |
| System.Workflow.ComponentModel | System.Activities |
| System.WorkflowServices | System.ServiceModel.Activities |
3.活动的变化
| WF3.x活动 | WF4活动 |
| CallExternalMethodActivity | Send |
| CancellationHandlerActivity | CancellationScope |
| CodeActivity |
CodeActivity/NativeActivity |
| CompensateActivity | Compensate |
| CompensatableSequenceActivity | CompensableActivity |
| ConditionedActivityGroup | 可通过其他方式实现 |
| DelayActivity | Delay |
| EventHandlingScopeActivity | 暂无 |
| FaultHandlersActivity | TryCatch |
| HandleExternalEventActivity | Receive |
| IfElseActivity | If |
| InvokeWebServiceActivity | Send |
| InvokeWorkflowActivity | 暂无 |
| ListenActivity | Pick |
| ParallelActivity | Parallel |
| PolicyActivity | 暂无 |
| ReplicatorActivity | ForEach, ParallelForEach |
| SendActivity | Send |
| ReceiveActivity | Receive |
| SequenceActivity, SequentialWorkflowActivity | Sequence |
| StateMachineWorkflowActivity | 暂无 |
| SuspendActivity | 无 |
| SynchronizationScopeActivity | 无 |
| TerminateActivity | 无 |
| ThrowActivity | Throw |
| TransactionScopeActivity | TransactionScope |
| CompensatableTransactionScopeActivity | TransactionScope + CompensableActivity |
| WebServiceFaultActivity | 整合为工作流服务 |
| WebServiceInputActivity | 整合为工作流服务 |
| WebServiceOutputActivity | 整合为工作流服务 |
| WhileActivity | While |
转载地址：http://www.cnblogs.com/carysun/archive/2009/07/23/CustomeActivity.html