You’ll still pay all costs for a spell cast this way, including additional costs. You may also pay alternative costs if any are available.

If you don’t play the exiled card you chose, it remains exiled. The card you didn’t choose remains exiled whether you play the chosen card or not.

Magmatic Channeler gets just +3/+1 from its first ability, no matter how many instant and sorcery cards are in your graveyard beyond the fourth.

You choose which card you’ll be allowed to play this turn while Magmatic Channeler’s ability is resolving, but you must follow the normal timing permissions and restrictions for the card when you play it. If it’s a land, you can’t play it unless you have land plays available.