Casting Stalwart Valkyrie for its alternative cost doesn't change when you can cast it.

Once you announce that you're casting Stalwart Valkyrie using its alternative cost, no player may take other actions until the spell's been paid for. Notably, players can't try to remove creature cards from your graveyard to stop you from casting the spell.

Stalwart Valkyrie has mana value 4, no matter what you actually paid to cast it.