1.) Like every other USB device from us you can connect as many of them as you want to one PC. The software is able to differ between all of them since every device has another serial number which is used for the identification.

2.) No, there is and will never be a way to connect an aquaero 5 to an aquaero 4. It should be possible in the future to connect an aquaero 5 LT as slave device to an other aquaero 5 via aquabus.

3.) The poweradjust 2 only offers a regular 3-pin fan output with analog voltage control. By the way: if you want to connect it to the aquaero 5 you can go with the standard variant since the ultra variant will not offer any advantages in this combination. The ultra variant is useful if you want to use it via USB because this way you can use its temperature control options.

4.) Neither on aquaero 5 nor on aquaero 4 you can connect anything additional via USB. Other devices are connected via aquabus.

5.) I'm not quite sure if I got your question right. To shutdown the PC with an aquaero 5 you have several options. Since the aquaero 5 also uses USB keyboard functions it can "press" keys for standby, hibernate or shutdown. This way you would not need any accessories at all. With the relay on the aquaero 5 you can use two different methods: it is possible to connect the relay parallel to the power button of the case so the aquaero can "press" the button for x seconds and the PC will turn off, or you use the ATX break adapter cable so the aquaero would interrupt the a connection which tells the PSU to run and the PC would turn off immediately. In both cases you need the 3-pin relay plug and for the ATX break method the adapter cable.

6.)