can i cast soul feast if i have platinum emperion

[kartiv]

can i cast soul feast if i have platinum emperion

to make other player lose life

[Kenji]

Yes. Although your life total cannot change, you can still cast spells and activate abilities that include an effect that tries to change your life total.

On the other hand, you cannot cast spells or activate abilities that have a cost of losing/gaining life, since you would be unable to pay the cost.

[Piplup]

Doesn't this spell cost u 4 life?

[Kenji]

No. It doesn't "cost" anything. Read Soul Feast's text again:

Target player loses 4 life and you gain 4 life.

Even ignoring the fact that it actually gains you life, there's no additional cost to casting this.

Compare with Withering Boon, which states:

As an additional cost to cast Withering Boon, pay 3 life.

And, as established before, you can't pay life for costs if you have Platinum Emperion, which its Gatherer ruling entries make quite clear:

1/1/2011: You can't pay a cost that includes the payment of any amount of life other than 0 life.