It doesn't matter, because if you prepare /dev/synoboot2 to 713+ your installed DSM thinks you placed the HDDs in a new DS713+.
Just SSH after upgrade to your NAS and enter:
sudo sed -i 's/maxdisks="2"/maxdisks="4"/' /etc.defaults/synoinfo.conf
sudo sed -i 's/internalportcfg="0x3"/internalportcfg="0xf"/' /etc.defaults/synoinfo.conf
and reboot to enable all 4 bays
I found a cheap and easy way to get a f400-Stick
There are plenty usable sticks, which are cheap or free giveaways
Just download "chipgenius", to identify the controller of your old USB-sticks.
Find one with a "Alcor Micro" controller (3 of 5 of my sticks had this controller)
google for: "Alcor Change PIDVID Rework 1.0.0.2" and download this old 32-bit executable.
You can change very easy VID and PID of Alcor sticks.
