QIf this is the only card on my field at the start of my main phase, can I select 0 other cards and place a prayer counter only on this card?

AYes, you can.

QAt the start of my turn, this card is on my field with 3 prayer counters. At the start of my main phase, I place a fourth prayer counter on it and it becomes a follower. Can it attack immediately?

AYes. It can attack both enemy followers and leaders.

QI have this card with at least 4 prayer counters on my field, and it has been given Ward by another card's ability. If it becomes Boxed via Nahtnaught, Cursed Queen's ability, does it retain Ward after it is no longer Boxed?

AYes, it does.

QI have this card with at least 4 prayer counters on my field, and its attack or defense has been modified. If it becomes Boxed via Nahtnaught, Cursed Queen's ability, what happens to its attack and defense after it is no longer Boxed?

AIt would retain the modifications to its attack and/or defense. Followers that become amulets and return to being followers retain their attack and defense values. Those values simply aren't referenced while they are amulets.

QI have this card with at least 4 prayer counters on my field. If it becomes Boxed via Nahtnaught, Cursed Queen's ability, is it still treated as a follower?

ANo, it would be treated as an amulet because its "While this has at least 4 prayer counters, it is a follower" ability would be lost.

QIf this card has fewer than 4 prayer counters, can it attack?

ANo. Amulets can't attack, be attacked, or take damage.

QIf this card has fewer than 4 prayer counters, can its attack and defense values be referenced?

ANo. It still possesses attack and defense values, but you treat the card as if it doesn't have them, and they can't be referenced by abilities.

QIf this card has at least 4 prayer counters, do effects which target amulets apply to it?

ANo, they don't.

QI have a Whitefang Temple with 2 prayer counters on my field. At the start of my main phase, can I first resolve this card's "At the start of your main phase" ability and place a third prayer counter on Whitefang Temple, then resolve Whitefang Temple's "At the start of your main phase" ability to search my deck for a Faith follower that costs 4 or less and summon it?

AYes, you can.

QIs this card still an amulet while it has at least 4 prayer counters on it?

ANo, it is purely a follower.

QCan I search my deck for and summon this card with Rodeo, Anathema of Judgment's On Evolve ability?

AYes, you can.