Sourcetree lists P4Merge as a merge tool available to use (under Options -> Diff).

However, after setting the merge tool to P4Merge, and then setting the merge command to the filepath for P4Merge.exe, I am still unable to resolve conflicts by using Context Menu -> Resolve Conflicts -> Launch External Merge Tool.  

The "Visual merge in progress" dialog opens for a moment, and the *Base, *Local *Remote etc. files are created for a moment, but then the dialog closes and the files disappear.   P4Merge never opens.

When setting the Merge Command for P4Merge under Options -> Diff, I've tried using both the full filepath of P4Merge.exe without double quotes and the full filepath of P4Merge.exe with double quotes, and neither work.