Being in the prone position has some advantages. You're harder to hit with melee and ranged attacks. But what is considered prone for D&D 4th ed. My rogue uses hand cross-bows. So could he go prone using a kneeling position (like using a rifle) and gain the benefits for being prone, or is that more of the Squeeze move act? Are there penalties to making ranged attacks while prone and feats to counter them?
The complete rules for the prone condition appear in the Rules Compendium, pp. 232–233.
You can voluntarily drop prone as a minor action, but you probably don't want to in normal circumstances. D&D uses the condition to represent falling down in awkward or unsteady ways, not controlled crouches like a rifleman's stance. The only advantage of falling prone is a +2 bonus to defenses against distant ranged attacks. There is no defense bonus against melee attacks: On the contrary, prone characters grant them combat advantage. There is a –2 penalty to all attack rolls while prone.
There are many feats and powers that knock foes prone, but very few mitigate the disadvantages of being prone. Low Crawl from PHB3 allows you to shift while prone, and Keep Your Feet from Psionic Power allows you to stand up as an immediate reaction. Grounding Shot from PHB3 negates the penalty for attacking prone targets, but it doesn't help when you're the one who's prone.
First, here's a list of what exactly Prone does:
The creature is lying down. However, if the creature is climbing or flying, it falls.
The only way the creature can move is by crawling, teleporting, or being pulled, pushed, or slid.
The creature takes a -2 penalty to attack rolls.
The creature grants combat advantage to attackers making melee attacks against it, but it gains a +2 bonus to all defenses against ranged attacks from attackers that aren’t adjacent to it.
Of those effects, the only good one (in most situations) is the +2 bonus to defenses against non-adjacent ranged attacks. Other than movement based issues the main downsides to being prone are granting Combat Advantage to adjacent enemies and the -2 to attacks.
However, all is not lost! A length of rope and most of your prone problems melt away!
Rope of Slave Fighting:
Level 7: You do not take the –2 penalty to attack rolls while prone.
Level 17: You also do not grant combat advantage while prone.
It uses your waist slot, but the level 7 version takes care of the most important bit for choosing to fight from prone as a ranged attacker. Hopefully when you (willingly) decide to drop prone it's not because you're about to mix it up in melee. The level 17 version makes Prone only a problem for moving around. Either way, depending on your play style, these may be perfectly reasonable sacrifices for the +2 to defenses against ranged attacks. As this is not cover or concealment, you will likely also be able to stack it with such in situations where it can apply without blocking line of sight/effect, though I'd check with your DM just to be sure.
I built a cleric for a one shot once that would drop prone with one of these belts while focusing on ranged attacks and conjurations and it was quite enjoyable.
For the movement side of things, as Bradd Szonye's answer and comment mentioned there's the feat Low Crawl to shift while prone. Also look at the level 2 Acrobatics Skill Power Agile Recovery which allows you to stand as a minor action At-Will. This will allow you to get out of prone, move, and attack in the same turn if you need to, thus making it so once you can afford the level 17 version of the Rope of Slave Fighting the only real general downsides* to dropping prone are the minor action to drop prone and stand up, and the inability to take advantage of movement that may be granted by other party members while you're prone.
*Some monsters have bonuses which trigger if their target is prone, but those are explicitly called out and not inherent to the prone status.
Being in the prone position has some advantages. You're harder to hit with melee and ranged attacks. But what is considered prone for D&D 4th ed. My rogue uses hand cross-bows. So could he go prone using a kneeling position (like using a rifle) and gain the benefits for being prone, or is that more of the Squeeze move act?
Something to bear in mind is this: unlike previous editions, D&D 4th edition is not interested in simulating reality. It has chosen mechanics which support a balanced and fun game, and where some things seem weird or don't make sense (like a gelatinous cube being knocked prone) it requires you to use your imagination to figure out how things make sense.
So in D&D 4th edition, voluntarily applying the prone condition to yourself just applies a set of mechanics, and you could imagine it like Bradd described: being knocked down in an awkward, unbalanced, vulnerable position. In almost all circumstances, having the prone condition applied is not desirable - and it won't help you aim.
So what if I want to lay or kneel down and take aim carefully?
That's fine, and you can do that, but the game doesn't attempt to model that mechanically. It's just roleplaying, and it doesn't have anything to do with the prone condition.
You could consider this, roleplaying-wise, what might happen when your rogue uses a power that gives you a bonus to attack (if you have one). Creating your own flavour for how your powers work is totally acceptable, and I think the Player's Handbook might even suggest or encourage it where you feel you want to do this.
Alternately, you could also tell your DM you want to do this - I'd consider, say, giving you a +2 to attack if you use up your move action as well as your standard for the attack (representing you taking your full turn to carefully aim and fire), but this is entirely made up.
What about being harder to hit with melee and ranged attacks, and other advantages?
Simply: The game doesn't care. This isn't part of its mechanics. If you have defensive powers or a streak of good luck, feel free to express it in terms of crouching low to avoid attack. However, saying "I crouch down low" doesn't mean anything on its own, mechanically.
Unless you voluntarily give yourself the prone condition, in which case you'll get a bonus defence against distant ranged attacks, but you'll have all the other effects of being prone applied, too.
Aside: Wait, did you say laying down has nothing necessarily to do with being prone?
Yes. Laying down doesn't automatically apply the prone condition, but it's reasonable to expect most of the time it will. In terms of rules, the game doesn't really care to go into this much detail.