re: Poker & Am I raising for the right reasons preflop?
You should always have a specific reason for every move you make in poker. Even folding pre-flop has a reason - to protect your stack.
You are right on target with your reasons for raising pre-flop - you want to narrow the field; and you want to improve your chances of putting any callers on a range of hands.
If you are in a game with a lot of raising, preflop or otherwise, you are also motivated to raise if you want to build a big pot because you have a had with great potential.
In your specific example with an opponent that called your pre-flop raise and you have a flop of 429, the odds
are good that your opponent missed the flop. Of course, you probably missed it, too. That's a great spot for a continuation bet. If your opponent missed, he'll fold. If he calls or raises your continuation bet, he may have hit a set of 4s, or perhaps he likes to play 35s and is now sitting on an open ended straight draw. Of course, he could also be a pretty savvy player and knows you missed and knows you're only making a continuation bet and raises you to win the pot.