So at the price point of the 818, none of the receivers are capable of driving 4 ohm speakers adequately & not just the Onkyo?
Objectively, you can rig up the receiver in question with some THX AV speakers and turn it up to Reference level for three hours - you'll soon find out if the ultimate abilities of the receiver are good enough and up to the job. Many manufacturers will give 4ohm power ratings for receivers and amplifiers, whether or not they can actually drive them properly or not. I've seen one box mini systems as well as big standard hi-fi amplifiers with 4ohm power ratings in their specs - in reality, they'd struggle very early on.
At average volumes, and with a high-ish efficiency speaker, a budget amp will probably sound fine on the face of it, but once the going gets tough, it'll soon show signs of strain or lack of control. I remember the good old B&W DM601's were perfect for demonstrating this - if they sounded bassy, soft, dull, and lacking definition, the amp wasn't up to the job - put on a more stable amplifier that could deal with 4ohm speakers, and suddenly you had a well defined, punchy sounding speaker that was head and shoulders above anything else.
I think part of the problem is that more people have heard (or own) speakers that aren't being driven properly than people who have heard (or own) speakers being driven properly.
To answer your question, in my opinion, there are no £1k AV receivers that can properly drive good quality, 4ohm loudspeakers.