What’s the difference between IV and V?

The IV chord is built on the fourth note of the key. And, the V chord is built on the fifth note of the key.

What does i, ii, iii, iv, v mean?

In roman numerals, alphabets are used to represent the fixed positive numbers. These roman numerals are I, II, III, IV, V, VI, VII, VIII, IX, and X represent 1, 2, 3, 4, 5, 6, 7, 8, 9 and 10 respectively. After 10, the roman numerals are followed by XI for 11, XII for 12, XII for 13, … till XX for 20.

Is IV larger than V?

The numerals for 4 ( IV) and 9 ( IX) are written using subtractive notation, where the smaller symbol ( I) is subtracted from the larger one ( V, or X), thus avoiding the clumsier IIII and VIIII. Subtractive notation is also used for 40 ( XL), 90 ( XC), 400 ( CD) and 900 ( CM).

Why is the 1564 progression so popular?

It's diatonic The progression is entirely consonant, which means that it's very easy to listen to. Considering that the genre we're talking about is Pop, this decision makes a lot of sense.

The I–V–vi–IV progression is a common chord progression popular across several music genres. It uses the I, V, vi, and IV chords of the diatonic scale.
IV/V is the fourth diatonic triad of the key’s dominant (four of five, not five of four). For one thing, it’s a secondary predominant, not a secondary dominant.
In music, I–IV–V–I or IV–V–I is a chord progression and cadence that, “unequivocally defines the point of origin and the total system, the key.