There is no product named Fireface+. I assume you mean the UFX+:
https://www.rme-audio.de/de_fireface-ufx.html

You connect the Fireface UFX+ to PC or Apple using either USB3 or Thunderbolt.
On the PC take care that you use an USB port with a supported chipset, see UFX+ manual ch 40.3 p. 114f

The four Octamic XTC can be connected to the UFX+ through MADI using OM3 or OM4 multimode fiber cable in a serial fashion, the maximum cable length between each of the devices can be up to 2km.

You connect every device in a serial fashion:
UFX+ MADI OUT to XTC#1 MADI IN,  XTC#1 MADI OUT to XTC#2 MADI IN, etc
and finally from the last device XTC #4 MADI OUT back to the UFX+ MADI IN.
You can use OM3 or OM4 patch cables, they are being offered from 0.5m up to around 50m length.
OM3 is fully sufficient, today the price for OM4 is nearly equal, so you can use OM4 as well.

If you have a structured inhouse cabling, then you need to check whether it is OM3 or OM4 and use that, do not mix OM3 and OM4 on the same connection between two devices.

If you cable the devices in a rack or room without structured cabling, then simply use OM3 or OM4 as you like. As this is fiber cable take not to bend the cables too much, eg around corners or in a rack.

I wrote a blog article about my setup of UFX+ and two XTC: https://www.tonstudio-forum.de/blog/ind … Cber-MADI/

I recommend you to configure
- Delay Compensation on each of the Octamic XTC and setting it to Auto-CA, see XTC manual
- Auxdevice in TotalMix FX then you can remote control the XTC and save/recall the settings in TotalMix FX snapshots
(the blog article contains examples how to configure the XTC and TotalMix FX).

An alternative to the Octamic XTC is to use two or three 12Mic. The 12Mic has the preamps of the UFX+, can later be used also in an AVB environment, has a little bit lower per port costs, because it has 12 preamps in one RU.
But you can't use Delay Compensation (because 12Mic breaks the MADI 8 port schema) and Auxdevice support is still promised since longer time, but much delayed because of corona issues.

I am running currently a mix of UFX+, 12Mic and XTC and wrote a blog article about it:
https://www.tonstudio-forum.de/blog/ind … XTC-DE-EN/

Reasons to choose the older XTC
- Delay compensation
- Auxdevice (remote control through MIDI over MADI)
- more features (format converter, ADAT, AES)
- USB CC mode, possible to use it with an iPAD

Reasons to choose the newer 12Mic
- newer preamps of the UFX+ with 75dB gain range
- little lower per mic port costs
- AVB ready device that can use it's MADI port to integrate MADI devices into an AVB setup
- power supply redundancy (missing on the UFX+ though ...)

As Auxdevice for 12Mic is not yet ready you would need to operate the 12Mic through Webbrowser, so you need to connect each of the 12mic to a LAN to a Gigabit copper port.