/Silent should indeed work (I just tested this) with the normal offline installers.
Though, it will
not work with the current webinstaller.
However, note that it isn't completely silent anymore since
a webpage will be opened at the end of the install to thank the user for installing (I suspect this is the wrapper's fault - more on that below).
Plus!4 being installed or not shouldn't matter.
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Another parameter you should be able to use is
/Destination=directory.
And then there is
/? or
/Help or
-help.
However, this parameter apparently doesn't work anymore*.
* That is, it still works but only on the actual setup file itself. It does not work on the wrapper of the setup file. And it is this wrapper which is actually downloaded from the official download page as being the so called 'setup'. But thus in fact, the real setup is wrapped within it. Hence there is a problem/bug in the wrapper as it doesn't pass on the command line parameters to the actual setup. At least not the
/help parameter; The other parameters are being passed on.
Note: Plus!4 didn't had an extra wrapper around its setup like Plus!5 has.
Note: You can find the setup file in a subdir in your temporary files folder when you start the Plus! installer.
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** The webinstaller has some other command line parameters as well, but those are only used internally though.
/ 
