From the official 2001 Uno rules (emphasis in original),
Wild Draw 4 card... You can only play this card when you don't have a card in your hand that matches the color of the card previously played.... If you suspect that a player has played a Wild Draw 4 card illegally, you may challenge them. A challenged player must show his/her hand.... If the challenged player is guilty, he/she must draw the 4 cards. If the challenged player is not guilty, the challenger must draw the 4 cards, plus 2 additional cards. Only the person required to draw the 4 cards can make the challenge.
I was playing a game of Uno recently where this came up in my mind. I had a Wild Draw 4 in my hand and strongly believed that my opponent was likely to go out if I did not play it right then, even though I did have a matching color card in my hand. I decided to bluff and play the Wild Draw 4. My opponent did not challenge, and I went on to win the trick.
In retrospect, I've had a nagging feeling that what I did was not bluffing, but outright cheating. I'm not sure if my behavior was more akin to playing non-words in Scrabble (legitimate bluffing) or moving my opponent's chess pieces while they are in the restroom (cheating). The mention of a specific in-game challenge process and in-game penalty leads me to believe that my behavior falls under bluffing rather than cheating, but the wording of the rule ("You can only..." and "illegally") seems to imply that the rule writers attach some level of morality to the rule.
Is playing an "illegal" Wild Draw 4 considered a legitimate form of bluffing or an act of cheating? For example, if I was caught doing this at a tournament, would my entire penalty be drawing four cards or would I also face the possibility of formal censure or tournament expulsion for willful cheating?
By analogy, I could compare this with law, where there is generally a distinction between civil wrongs (in which the offender's entire penalty is compensating whomever they hurt) and crimes (in which the offender often faces so-called "collateral" consequences for months, years, or life as as result of having a criminal "record"). So, in terms of Uno, is an illegal Wild Draw 4 player treated more like "You played that card illegally, so you have to draw four cards. Carry on then." or more like "You played that card illegally! Draw four cards! Also, you're a cheater and are banned from further Uno tournaments at this location for the next year! Your behavior will be reported to the Disciplinary Committee of the Regional Council of Uno Tournament Organizers for a possible lifetime ban! Get off my property!"?
To be clear, I am asking for the closest I can get to a "rules as written" or "rules lawyer" answer. I recognize that one can house-rule this to their heart's content.
If a more recent ruleset clarifies this situation, quoting it is a legitimate answer.