Remove a node from Failover Cluster
Removing SQL Server from failover cluster - you have to run the setup.exe and in maintenance you have to select remove node.

Then removing the node from windows cluster, I would suggest you use PowerShell 

Remove-ClusterNode –Name node4 #change your node name here 


  The Remove-ClusterNode cmdlet removes a node from a failover cluster. After the node is removed, the node no longer functions as part of the cluster unless the node is added back to the cluster. Removing a node is also called evicting a node from the cluster.


The above commmand will ask your confirmation if you want to really evict the node or not -- unless you use use /force switch.