The 4.0.6 client is now the stable client with improved performance, lighting, and many other features.
A low memory version is also available and can be found here:
https://old.wurmonline.com/client/wurmclient_lowmem.jnlp
If you have issues please report them here:
https://forum.wurmonline.com/index.php?/topic/157281-stable-40-client-bug-megathread/
NOTE: If you ran your client with a batch file with memory modifiers PLEASE DO NOT USE THIS. The new client has a different setup and running this is likely to cause crashes and other conflicts.