Does Sona's ability target runes, and if so, does it require exactly 4 friendly runes to be on the board for the ability to trigger?

Ruling: Under the original card text, Sona's ability targets 4 friendly runes and cannot be put on the chain if there are fewer than 4 friendly runes on the board. The runes do not need to be exhausted - you can target already readied runes. The revised card text now says "up to 4 friendly runes," which allows the ability to trigger with fewer than 4 runes.

Sequence:

• At the end of your turn, if Sona is on a battlefield, her triggered ability checks if there are 4 friendly runes on the board
• If there are 4 or more friendly runes, you must target 4 of them to put the ability on the chain
• Upon resolution, ready the targeted runes (even if they were already ready)
• With the revised "up to 4" text, you can now trigger with fewer runes

Nuances:

• The ability targets because it affects specific game objects of the player's choice, not selected by objective criteria
• You need all valid targets to put an ability on the chain - "do as much as you can" only applies during resolution, not when entering the chain
• The runes being exhausted or ready doesn't matter for targeting purposes
• Mount Targon's Peak works similarly but requires 2 runes total (not necessarily friendly or exhausted)