The difference between 4-4 and 5-5 in the Majors

When you are 5♠-5♥ you will bid both your suits. It pays to open 1♠ so you can next bid 2♥. That is more economical than opening 1♥ and rebidding 2♠ (you’ll never be able to bid 1♠ after opening 1♥ because the bidding will have gone past that point).

 Say responder has a weakish “dustbin” 1NT response with three cards in the suit you bid first and two cards in your second suit. Contrast these two routes:

• a)    1♥ -  1NT - 2♠
• b)    1♠ - 1NT - 2♥

In (a), responder has to go to the Three-level to give a preference back to the first suit, 3♥.
In (b), responder can give a preference back to the first suit at the Two-level. 2♠.

When you are 4♠-4♥, you are not planning to bid both your suits. You’ll bid one, then rebid Notrumps (I’m assuming you’re 15-19, for with 12-14 and 4♠-4♥, you’ll open 1NT ). It’s better to open 1♥, because that gives partner the easy opportunity to introduce spades at the One-level. If partner doesn’t respond 1♠, you’ll presume he doesn’t have four of them and rebid Notrumps. Mistakenly open 1♠ and partner may lack the strength to respond 2♥. A heart fit might go a begging.