When I installed it, it asked what target app I wanted (I chose Daz Studio rather than Poser), then the installer chose /Applications/Poser 4 as the installation folder.
That “the installer chose” bit is, I think, actually a drop-down list where you can choose one of your possible default locations. In your case, “Poser 4” just happens to be the one that shows first if you don’t change anything. Click on the little down-arrow at the end of that line in the installer, and you should have your D|S content (whatever you’ve called it) as one of the options.
Some installers are more finicky than others, depending on how old they are, but once you’ve done this a few times it should stay as your “first preference” location choice.