My opponent uses Powerful 4 on an attack discarding his only 2 momentum to give the attack +8 damage, and I respond with Spiritual Center. Does Spiritual Center cancel out the whole +8 damage bonus or only 4 damage?

Spiritual Center

R Commit: When an attack is getting a speed or damage bonus, if that bonus is equal to or greater than your opponent's momentum reduce that bonus to 0.

401.2.2 Powerful: X – E Discard any number of momentum: For each momentum discarded to pay for this ability, the powerful attack gets +X damage.

it cancels all of it

Stamp for protoaddict.

The boost from powerful isn't "+4, +4" it's just "+8".

Note that your opponent would have to have at least 11 momentum, before the enhance in your example, for spiritual center to be ineffective. If he has 10 momentum, he'll discard two, and 8(momentum) is equal or greater then 8(damage) so SC will work.

At 11 he discards two, has nine left. 9(momentum) is not equal or greater then 8(damage) and spiritual center wouldn't work.