Can a player tap 4 runes for a unit, recycle one tapped rune to play Seal, then use that Seal (by tapping it) as part of the cost for the unit they're playing?

Ruling: Yes, this is legal. Tapping runes adds them to the rune pool, which can be used flexibly throughout the turn. A tapped rune can be recycled for costs, and gear like Seal enters ready and can be immediately tapped to contribute to power costs.

Sequence:

• Tap 4 runes to add them to the rune pool
• Recycle one of the tapped runes to pay for Seal
• Seal enters play ready
• Tap Seal to add to the rune pool
• Use the rune pool (including the contribution from Seal) to pay for the unit

Nuances:

• The rune pool persists throughout your turn and only empties during draw phase or end of turn
• Gear enters play ready and can be tapped immediately
• This type of play is commonly used with Darius