yea idk about mastery lvl 4. maybe mastery lvl 1 or 2. But people should NOT be able to first time a champ in ranked, thats ridiculous. And goes into the category of "unlucky" elo loss. I'm absolutely ok losing games due to my team getting outplayed, but when i lose a game partially due to someone spending 20% of the game tabbed out looking up their build, the rest of the game feeding because they don't understand anything about the champ let alone the intricacies, such as certain lvl/item power spikes. I mean ranked SHOULD be taken a little seriously. I get it's just a game, but in a competitive environment when you have half the players with a great desire to win, and the other half just playing whoever the heck they feel like treating ranked no different then they would a normal, I think that breeds a lot of really avoidable toxicity. And I get there is a PBE and all, but is it really too much to ask a player to prove their worth on a new champion with a few normal matches. I see so many wasted bans on new champs just so THEIR OWN TEAM won't play them. And this already creates a toxic environment. Someone got their champ banned, and some teammates already assume the guy is a troll because they wanted to play the new champ in ranked an hour and a half after release.
And to one other persons comment about trading, have trading be something specified during ban phase, it would then allow you to pick from the other players pool of champions, and then allow them to pick from yours. then it would auto trade once pick phase is over. A really simple fix, that honestly solves another problem of "afk i forgot to trade" as well as many times riot has said they like trading as a "tactical" option available. But needing to "own" the champs takes away a lot of that, so often the first/last picks dont have each others champs and it takes away that "tactical" option. So allowing a trade to be specified in ban phase, and choosing from each others champ pools would also solve this problem as well, making trading an actual "tactical" option, as it is meant to be.