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I have taken the fstop loka within the usa even on regional jets as a carry on.
The medium pro icu is great. It will fit a 400mm f4 horizontally. I still manage to fit my 5D, a couple lenses, filters, chargers, ballhead, vixen polarie into the icu. In case I have to take the icu out and put it under the seat, I do not pack anything expensive elsewhere in the Loka. That being said, even in the regional jet I was on, the Loka would fit under the seat in front of me.
The 500mm f4 plus some small amount of other gear will probably require the large icu and vertical placement. You'll have to review the specs to see if it would fit. F-stop has sample pictures of various gear stuffed in their icu's but the largest I see is a 300mm 2.8 which is about the same size as the 400 f4.
They have a responsive support system for inquiries, you should contact them.
It is very comfortable to hike with all day.
As you can tell, I love the bag.