It could easily still be luck if they bought Tesla or TECL 3 years ago and that is where their 4x return came from.

Regardless, even if you have a strategy that works for now, that doesn't mean it will continue to work forever. Anyone investing that isn't an expert in a particular industry in which they trade is basically just guessing. Any insight they have will almost invariably already be priced in.

>It's not luck if you make consistent returns over 4 years.

Sure, if your sampling was unbiased. But here, it's not - so that user might well have been lucky. How many people have failed to make those returns, or any kind of return?

There's very obvious evidence that somebody can outperform the market, even to a massive degree. From what I know, there's very little evidence that a particular person can deliberately outperform the market - and daytraders, especially, cannot do so consistently.

Respectfully, 4 years is still luck. You need to maintain that for 20-30 years (or be VERY lucky and make enough in a super short time to exit the game).

you can measure how risky it was using something like Sharpe Ratio. Then you can assess how likely you are able to repeat it again. Also, kelly criterion allows you to measure risk of ruin for every bet. "making consistent returns in 4 years" does not really mean anything without additional information.