Conception: "Don't Open A Weak 2-bid with a side 4-card major"
The Truth: Preempts can be made with a variety of shapes.

Players love rules. Bridge teachers sometimes give in to the temptation to give players lots of them. I can't tell you how many bad things I hear after a sentence starts, "My Teacher told me ..."

The truth is that preempts are entirely too complicated to describe with rules. None of the following are good rules: "Don't preempt with a void" or "Don't open a weak 2-bid with a side 4-card major" or "You need 2 of the top 3 or 3 of the top 5 honors in the suit." So many rules. So much nonsense. 

Deciding what a preempt is depends a lot on seat and vulnerability. In first seat, favorable (not vulnerable against vulnerable), you might catch me opening 2 with as little as 

 KJ9876  
8764  
 8 
 72
 

However, if you make my side vulnerable or had my RHO passed first, I wouldn't dream of bidding. Similarly, third-seat preempts can be very wide ranging. Since you don't worry about partner having good values, you can easily preempt with a void or a four-card suit. Also, everyone has their own personal style (aggressive or passive). Most successful tournament players are aggressive preempters.

If you ask an expert for rules, they'll tell you that their preempts should resemble what partner expects given seat and vulnerability. They paint a picture of your hand. So if your hand is "weak with hearts/spades/diamonds" and that is what you want to convey, then open 2 or 3 of that suit. How weak this can be, as I've said (I repeat myself as I get older) depends on seat, personal style and vulnerability. But a side void, a side ace, a so-so suit, a side four-card major -- all possible.  Here is one that violates all the "rules" -- Pass-Pass to me, at favorable vulnerability, I would consider it winning bridge to open 2 with: 

 Q109765  
 J765  
 A32  
 --.
 

If you pass in this situation, you make it too easy for your opponents.