Consider this scenario, if the receiver is a prospect purchase and you want details about its performance before you buy and you visit a FAQ for the unit in question. The question about lower loads is a common one and since Denon would like for you to purchase their products, not NAD or Rotel.
Then you purchase the product slightly based on the information you find in this FAQ. The Denon has a warranty and you call or email Denon to find out if it is all-right to use your 4 ohm speakers with the receiver, they say, "NO!, that is a terrible idea!". Why would they do that you wonder?
Denon doesn't want to repair the unit while it is under warranty, because it costs them money.
Truth of the matter is, they should probably work fine. The Denon may not be stable on 4 ohm loads at higher levels as the amperage requirements will jump at lower frequencies. Ohms is nominal, not fixed, so the speaker could conceivably dip to 2 or 1 ohms, and at higher volumes could cause the amp to go into protect.
Given that you only use the receiver at moderate levels there shouldn't be a an immediate threat, but I wouldn't continue using the receiver to power the Onix Ref. 1s. I would start saving for an amp, if you have to get a smaller amplifier to conserve space Outlaw has some small power amps that work comfortably with difficult loads.