Hi everyone!
Hoping I can get feedback from those who are using the GPeRx4.
Mikrotik mentions as the only PoE-IN option: 802.3 bt.
In my usecase, I would have 2x 17W cameras downstream of the GPeRx4, so a PoE-IN 802.3 at injector(60W) would clearly cover my need.
Where I am PoE-IN 802.3 at injectors are way easier to find and way cheaper.
My question is if such PoE-IN 802.3 at injector would be able to power the GPeRx4 with 2x 17W downstream.
Many thanks!
No, it seems like the GPERx4 needs "bt". (while GPERx6 can work also with af and at)
See here:
BUT you have some data mixed up:
https://en.wikipedia.org/wiki/Power_over_Ethernet#Standard_implementation
in a nutshell:
802.3af (type 1) = 15.40W (at PSE), 12.95W (at PD)
802.3at (type2) = 30W (at PSE), 25.50W (at PD)
802.3bt (type3) = 60W (at PSE), 51W (at PD)
802.3bt (type4) =90W (at PSE), 71.3W (at PD)
An "at" injector delivers max 25.50W your cameras want 2x17W=34W, even if the 17W are "max" power and in the real world the devices will use a little less power than that, 25.50/34=0.75 are very likely not enough.
To power them you do need a "bt" PSE (it can be of type3 for the power requirement).
What remains unanswered is whether the Gperx4 will work also with a type3, whilst the docs/page talk of 90W, so type4.
SInce they talk of" bt" without specifying "only type4" it would make sense that it can run also with type3, but it needs to be tested and confirmed.
Many thanks for referencing the other related topic and also for making justice to the 802.3 hierarchy for me 
After reading it, I understood that actually Mikrotik confirmed that the 802.3bt is not even required for PoE-IN, as long as the total consumtion of the extender does not exceed the source:
“MikroTik confirms, as long 802.3at/af does not exceed watt usage for all powered devices including GPERx6 itself, it can be used. 802.3bt is not mandatory for PoE-IN.“
Yep, but that applies to the GPERx6, NOT to your GPERx4, this latter NEEDS 802.3bt according to the report found of Reddit BUT the issue that remains unsolved is GPERx4 compatibility to BOTH type3 (60W) AND type4 (90W) or if it ONLY negotiates for type4.
Ok, I have tested this myself and confirm working properly with the following gear:
Works great and confirms 90W IN are not mandatory, but 802.3bt, 60W are good enough.
Very good 
.
Another small confirmed datapoint to add to the knowledge base.