Pick a game. There are dozens of different video-poker games in casinos. Different games will have different rate of returns, meaning some games, if you win, will give you a higher percentage of your money back and others will give you a lower percentage. A "9/6 Jacks or Better" paytable is the best because the rate of return is 99.54%, meaning the casino only keeps .46% of your money.
Once I hit two 4 of a Kinds back to back. The probability of hitting it once is 0.24%. The chances of hitting it back to back are 0.24%2 = 0.0006%, or 1 in 179,000. That's four times less likely than hitting a royal flush. And I would have preferred the royal! (By the way, purists will note that after you've hit a 4 of a Kind, the chances of hitting another four of a kind is just 0.24%, not 0.0006%, but I'm considering the probability of getting two back-to-back before I get the first one.)
Video poker is a very volatile game, about four times as much as blackjack. In any form of gambling, short-term results mostly depend on normal mathematical randomness (what some might call luck). However, in the long run, results mostly depend on skill. If you play a game with a return of 100.76% perfectly, that does not mean that you will have a 0.76% profit every time you play. The 100.76% is an EXPECTED return. Much in the same way, if you flip a coin ten million times, the expected number of tails will be five million, but it is unlikely you will hit five million on the nose. Actual results will vary significantly from expectations, but the more you play, the closer your actual return percentage will get to the expected return.
Despite the importance of finding the best machines, most players don't. That's why casinos can offer both decent and lousy machines in the same casino and be confident that gamers will still play the lousy ones. They have to keep some good machines, otherwise they'd lose all the players who know what they're doing. But most of the machines will be bad, and most gamers will play them anyway. Heck, in Vegas even casinos and supermarkets have video poker, with absolutely terrible paytables, but people will still play them rather than going across the street to a casino where they can get seven times better odds. Go figure.