With (i) you would open 1 ♥; however you would not rebid 2 ♦ over, say, a 1 ♠ response. You would rebid 1 NT (15-16 balanced). The only time you would open 1 ♥ with just a fourcard suit is a balanced hand* and those hands will rebid notrumps. Therefore 1 ♥ - 1 ♠ - 2 ♦ shows a hand such as (ii) – five ♥ - four ♦.

Look at the ditty another way: you should not introduce a second suit unless you have five (+) cards in your first suit*.
Consider these three hands:

Holding (iii) you open 1 ♥ [the one exception to the open-higher-ranking-with-two-equallength-suits rule is 4 ♥ - 4 ♠]. If partner responds (say) 2 ♣, you must rebid 2 NT (balanced 15-16), not 2 ♠ – which would show five hearts.

Holding (iv) you open 1 ♠ and rebid 2 ♥ over 2 ♣, in spite of the poor suit quality, thereby showing your five ♠ - four ♥ shape.

Holding (v) you open 1 ♣ and rebid 1 ♠ over 1 ♦/♥: rebidding a cheaper four-card suit is preferable to repeating a six-card suit.

*Apart from the mercifully rare 4441 shape. We will discuss how to handle the awkward 4441s in a few pages time.

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