I finally got around to downloading FlowJo 10.4 this morning. Double clicked on the installer as usual, expecting a disk image to mount or something to happen:
After waiting around for a while, nothing seems to happen. It's possible that an embedded agreement is preventing the installer from opening.
There's an easy fix for this using hdutil tool on the OSX command line (terminal):
hdiutil attach ~/Downloads/FlowJo-OSX64-10.4.0.dmg
You'll be presented with a compatibility warning agreement on the command line that looks like the following:
Simply press the q key to dismiss the agreement, then Y to agree:
The .dmg should mount fine, and you'll see it in the finder.