As Bruh said, the Virtuoso is a bit different from the other characters.
All your abilities are usable once you unlock them, you're not limited to 4 like the other characters. All you need to do is to use the 1, 2 or 3 keys to enter the song's "notes" and press 4 to play the song.
Example:
Hot Licks is 3 x Fire notes, so you press 1, 1, 1 and then 4 to "play" the song. Easy!
Then if you want to go for Power Chord, which is 2 x Fire notes + 1 x Lightning note, you press 3 (for Lightning) and then 4 to play the song.
Notice that I didn't enter the first 2 "Fire" notes for the Power Chord song. That's because they're already present on your UI bar from the previous Hot Lick song. As long as the required notes are entered in that bar, you're good to go since the order doesn't matter. This allows to save time because you don't always need to play all 3 notes to play the song you want.
When you get more comfortable with that concept, you'll be able to start "chaining" songs. "Chaining" songs simply mean you don't have to enter all the notes before playing the songs because the notes from the previous song can be used for the next song. The Hot Lick + Power Chord "chaining" is a good example. After Power Chord, you could chain Battle of the Band by simply pressing 3 then play it with 4.