Can you partially pay a cost with energy, then use Hextech Anomaly's effect to pay the remaining cost?

Ruling: Yes. You can use reaction-add abilities (like Hextech Anomaly's) anytime when paying for costs, though you still pay all the costs at once.

Sequence:

• Exhaust 4 runes for 4 energy
• Recycle those runes for 4 power
• Use Hextech Anomaly to convert that power into 4 additional energy
• Pay the total 8 energy cost at once

Nuances:

• The recycled runes produce power of a specific domain, but that power can be used to pay for Hextech Anomaly's ability
• All costs are paid simultaneously even though reaction-add abilities can be used during the payment process