How can I customize error message when running singleton rcp4 application second time? Now the default message is:
could not launch the product because the associated workspace is currently in use by another application.
The LifeCycle class @PostContextCreate method runs before this check is done so you can add your own code there to do the check and display your own message.
The checks done by Eclipse are fairly complex, you can see the code in the checkInstanceLocation method in the org.eclipse.e4.ui.internal.workbench.swt.E4Application class