It's one of the most-asked questions:

"When I'm 6-4 and open the 6-card suit, should I repeat the 6-card suit or bid the 4-card suit next?"

Bad news -- there is no "right answer."  There is no definitive rule or agreement among teachers and experts.

Here are some mainstream thoughts/guidelines:

With 6 of a minor and 4 of a major

• Open in the minor, of course, then try to show the major on Round 2, but not at the cost of reversing with a minimum hand.

For example, with:

 KJ96  
 2  
 K2  
 AJ7642
 

 Open 1 and rebid 1 if partner answers in a red suit. But, if partner responds 1NT, you aren't strong enough for 2 (which shows a good enough hand to be at the 3-level when partner likely goes back to clubs).  With: 

 8 
 A1086  
 QJ10765  
 AJ
 

 open 1 and rebid 2 after a 1 (or 1NT) response. You are not strong enough to reverse.

With 6 of a major and 4 of another suit

• With a minimum hand, tend to repeat the 6-card major.

            For example, rebid 2 after 1-1 with:  6   AQ10765   K765   K2.
 

• With extras, show a decent 4-card minor on Round-2, hoping to get a 3rd bid. On the 3rd bid, repeat the major and partner will expect a 6-4 with extra values.
  
  So, rebid 2 after 1-1NT with:

   K108654  
   A2  
   2  
   AKJ2.   Hopefully, partner takes a second call, and you can bid spades next.

• With 6 spades and 4 hearts, after 1-1NT, it is a good idea to "always" (even with a minimum) mention the hearts. There is a good chance of a 4 game (since responder can have lots of hearts for his 1NT response to 1). 
  So, after 1-1NT, rebid 2 with:

   AQ8765  
   KQ65  
   43  
   2.

Notes:

These are guidelines only; there is still room for judgment. Some players are strong believers that with a 6-card major and a 4-card side suit they should always bid the 4-card suit on Round 2. They feel it is important to show at least 9 cards in their hand (partner will know they are at least 5-4 in 2 suits) as opposed to only 6 cards (which is all partner knows when you repeat the 6-card suit on Round 2).

"Form of scoring" is certainly relevant. At matchpoints, where playing a higher-scoring partial is so important, the scales should tip more towards repeating a 6-card major (to avoid the risk of ending in a lower-scoring minor).

"Suit quality" is also a big consideration. Even though it is a minimum hand, I'd rebid 2 after 1-1NT with:

J76542  
 5  
 AKQ10  
 K2.   But, with:

 KQJ1076  
 A2  
 Q765  
 3,
 

I'd surely rebid 2 and eschew the diamonds.

"2/1 Auctions."  If the auction begins, for example, 1-2, especially if it is GF, there will be plenty of time to repeat a 6-card suit later (there will be a "later."). So, when in a GF, it is usually best to show the 4-card suit ASAP. 

What about 6-5? Click here to read more.