Quiniela boxes are a very good bet, both for beginners and seasoned handicappers. Whether you bet 3 or 4 dogs, if you're at least an average handicapper, you have a good chance of winning. There's one bad habit that can really wreck your quiniela record though, and way too many people don't even know they have it.
It's second-guessing and it usually happens after you've gone to the window and made your quiniela box bet. Let's say you bet the 1/2/3 quiniela box. You walk away from the window, looking at your program, and all of a sudden you realize that the 4 dog looks really good too.
This is when you have to make a crucial decision. Do you leave the 4 out of your bets, even though you're starting to think that it really has a chance and might even be better than the 3? Or do you go back to the window and bet some combinations with the 4 - just for insurance?
My advice is to sit down and watch the race and stick with what you handicapped in the first place. I have two reasons for this advice. One, your first impressions are more likely to be accurate when you handicap a race. If you "over-think" a race, you start seeing all kinds of possibilities and get to where it looks like any dog could win.
Two, if you go back and bet some combinations with the 4, you'll end up spending more money than most quinielas pay. But if you only bet a couple of combinations with the 4 - say a 1/4 and a 2/4 - it seems like fate almost always has the 4 come in with the dog you didn't pair it with in a quiniela. (I speak from experience here, unfortunately.)
So, make your quiniela bet and then stick to it. Don't second-guess yourself or look for "better bets" that you could have made. Almost always, you'll be glad you did.